From what I understand, it's formaldehyde. Nice, huh?
I once had a pair and the smell was there, no matter what I did. I got rid of them. I certainly didn't want them on my skin, far less polluting my house w/ toxic, cancer-causing chemicals.
P.S. Mine were form Target, Mossimo I believe. When I was checking out, the male cashier kept saying to me, "Make sure you wash these before you wear them, ok?" He must have said it 3 times. He was kind of young, so I just kind of brushed him off and smiled (of COURSE I wash everything new before wearing it, why does he keep telling me this?) Well, after the stench never went away, it made me reflect on the kid telling me over and over to wash the jeans :)