Lena Y. 5 I think it is silk. I won't try by burning because I don't want to destroy the item but I'm pretty good at telling silk from polyester and it doesn't feel like poly.Feeling when wearing is very nice! And it is very functional slip. Not static.Even when I wore white dress with this slip and black underwear, I couldn't see it through. (it would be very obvious without the slip). I really like it and it's a good buy.