A synthetic antioxidant often seen in skin-lightening products. Compared to proven skin lightening agents such as hydroquinone or forms of vitamin C, phenylethyl resorcinol has scant research behind it. The most compelling research looked at the results of a cream with phenylethyl resorcinol plus three other skin-lightening agents. The product was applied over a period of 3 months by 20 women, all of whom also used sunscreen. At the end of the study, it was determined that the women's dark spots decreased by 43%. The problem is we don't know how much of this improvement is due to phenylethyl resorcinol, as it wasn't used alone, which is the case with most products using this ingredient as a means to lighten brown spots (Source: Journal of Cosmetic Dermatology, March 2013, pages 12-17; and September 2011, pages 189-196). Until more research comes to light, this isn't an ingredient to bank on for great results if dark spots are your concern.