Yup, STEAM. If you have time to let the soup cool, do. Then reheat to serve. If you don't have time, try blending in batches, filling the blender about halfway. Use the pulse a couple of times (HOLDING DOWN THE LID W/A TOWEL AS SUGGESTED) and then take the top off and let the steam escape. Now try blending, starting at a slightly slower speed and ratcheting up if it's going well.

If your lid has it, remove the the cap in the lid used for pouring in liquids while running. Place a folded paper towel over the opening and then a towel over that. It should help reduce the expansion of heated air and soup while running.

Get an immersion blender & blend right in your soup pot. Easier to clean & much faster. They are often on sale for $20-30.

I second Aschalk - many (most) blenders have a lid with a little removable center part. Take that part out and cover the open space with a towel (I use a dish towel). This lets the steam out without blowing the lid off, and the towel doesn't even get that dirty.

I don't pour hot liquids into my blender anymore, I just use a stick blender. Works great and allows me a little finer control over the texture of the finished soup (I could never get just the right level of chunkiness in a blender).

STICK BLENDER! I got one for Christmas and it is SO much easier. Otherwise use a towel on top of the blender.

Always always always hold the lid down when blending anything.

Having had the lid on my blender blow off one too many times myself, I love my immersion (stick) blender for any hot liquid. If you like to cook at all, they are fairly inexpensive and worth the investment.

Immersion blender is the way to go. Less mess, easier to wash and we don't get scalded... After our blender jar broke, we decided to try the immersion kind and we love it. The compact size is a plus when you don't have a lot of space in the kitchen!

I just vent the little pour hole a little bit. It allows the steam to escape. No problems!

Immersion blender, but anything with starch--like beans or potatoes needs less blending. I'm more of a fan of the hand-operated food mill if I want to puree the whole thing.

I've had this happen to many too many times so now I just use an immersion blender. It didn't matter if I took the center out of my lid and placed a towel on top, only filled it up halfway or what, the soup sprayed every time. Immersion blenders are the only way to go.

Also you are only supposed to fill the blender half way (so blend in multiple portions if necessary) and then blend while holding the lid down with a folded towel.

You should absolutely get an immersion blender. Instead of pouring warm soup into a blender, you just stick your immersion blender straight in the pot.
You can also control more easily how blended your soup will be.
Good luck!

All of the above comments are correct. Taken out the removable center of the cover. Hold it down with a towel. Better yet, get an immersion blender.

But - you know how when you take water from the water cooler at work, a giant air bubble breaks the surface and explodes, and water gets everywhere (thankfully, contained all inside the 5-gal water bottle)? Same thing. You need to start your blender on low speed. You'll notice that the soup becomes a mini-tornado, and a vortex appears. If the blender is going to fast, the liquid will cover up the hole, and you'll get an air bubble. The air bubble will rise to the surface and you get soup everywhere.

So: remove the center of the lid. Hold a towel over it. Do it in batches. Start the blender on low, and slowly increase the speed. If the vortex disappears, you're going to fast - reduce the speed until the vortex re-appears.

Or maybe you shouldn't start the blender on full speed.

When the soup is really hot, blending it at full speed will incorporate more air into the soup. As the air heats (when it gets pulled down by the vortex of hot soup), it expands, leading to your soup spraying out everywhere. If the air is already fairly hot, it won't expand as much when it gets sucked down into the soup. So before you run the soup on full speed, pulse it a few times (I usually go about 5, starting with short bursts and slowly lengthening the blend time) on a low speed to warm the air up before you really liquefy whatever.

Start at the slowest speed and get the liquid into circulation, then increase the speed.

Immersion blenders are awesome for soup. I got mine for Christmas and it's much better. I can control the texture of the soup and it's much easier to clean than my blender or food processor and there's no mess as long as you keep it submerged. It also works like a charm.

Now my blender is relegated to frozen fruit smoothies and frozen adult beverages in the warm weather months, which is fine by me.

Yep, when stuff is hot...the explosion happens! Immersion (stick blenders ) are usually better for hot stuff, but if you have to blend hot stuff, do it slowly & keep your hand firmly pressing the top the whole time (no matter how hard u stuck it on before you u start blending). Since i make hot chocolate the Mexican way (by adding a disc of mexican chocolate into a pot with liquid and then letting it simmer...and then putting it into the blender to make it thicker, frothier, and blendededed)

So yes, hot stuff will always blow the blender cover off!

Seconding everything that everyone else has said, I've also had a glass blender break on me before and cut my hand badly because of hot soup. Still have the scar on my finger. Now I only do the immersion blender with hot liquids.