Synthetic antioxidant often seen in skin-brightening products. The most compelling research looked at the results of a cream with phenylethyl resorcinol plus three other skin-brightening 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 uneveness 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 improve the appearance of an uneven skin tone. Until more research comes to light, this isn’t an ingredient to bank on for great results if an uneven skin tone is your concern; however, it’s fine to use in products that also contain other skin-brightening ingredients with established track records of success. 
- Dreher F, Draelos Z, Gold M, Goldman M, Fabi S, Puissegur M. Efficacy of hydroquinone-free skin-lightening cream for photoaging. J Cosmet Dermatol. 2013;12(1):12-17.