What is the vapor when sugar boils without water?

[mrspeedybob]

I have a recipe that begins with sauteing onions in boiling sugar. It occurred to be today that I don't know what the vapor is that the sugar is giving off when it boils. There is no water in the pan at that point (other then what is in the onions). Are the bubbles that I'm seeing actually vaporized sugar molecules or does the sugar break down and release hydrogen, oxygen, water vapor, or something else?

 

[voko]

http://en.wikipedia.org/wiki/Caramelization