Retinol Ester.

"A313, which used to be called Avabon, is a combination of various retinol esters: retinol palmitate, retinol acetate, and retinol propionate, with retinol propionate being the most effective one. Retinol esters are the weakest link of the retinols and they need three steps from retinol esters to retinols to retinals to retinoic acids to actually get activated. So they're not that strong, but for whatever reason and I cannot fully explain it, maybe it is the polyethylene glycol base that this baby is in, it gives not only an instant gratification effect but it really does feel stronger than some of the retinols that are over the counter, especially when you're talking about retinols that are complexes and not actual percentages. With this guy in a dry dry climate because it is an oilier base, a little bit goes a very long way and you don't feel like your skin gets over stripped and dry. It is very tacky so it is not something I would tell you to put on your face right before going to bed because you will make your sheets all oily. Sometimes I get a little itchy when I use it but I kind of like that feeling. I still cannot use it around my eyes because I get irritated when I do. It does give you a smoothed out glowy appearance the next day."