All-trans retinoic acid (ATRA) plays essential roles in the normal biological processes and the treatment of cancer and skin diseases. Considering its photosensitive property, many studies have been focused on the photochemistry of ATRA. In this study, we investigated the transient phenomena in the laser flash photolysis (LFP) of ATRA in microemulsion to further understand the photochemistry of ATRA. Results show that 355 nm LFP of ATRA in both acidic and alkaline conditions leads to the generation of retinoic acid cation radicals (ATRA•+) via biphotonic processes. The employment of microemulsion system allows us to investigate the reaction of hydrophobic ATRA•+ with molecules of different polarity. Therefore, we studied the reaction activity of ATRA•+ to many hydrophobic and hydrophilic molecules. Results show that ATRA•+ can efficiently interact with lysozyme, tyrosine, tryptophan and many antioxidants, such as curcumin (Cur), vitamin C (VC) and gallic acid (GA). The apparent rate constants of these reactions were measured and compared. These findings suggest that ATRA•+ is a reactive transient product which may pose damage to lysozyme, and antioxidants, such as Cur, VC and GA, may inactivate ATRA•+ by efficient quenching reactions.