Fenton and Photo-Fenton processes are attractive alternate treatment method for to thiol compounds. The degradation of 4-mercaptobenzoic acid by Fenton and photo-Fenton processes was studied. The effect of various reaction parameters such as substrate, hydrogen peroxide, ferrous ion, pH, polarity of solvent and catalyst was studied. Photoproduct was characterized by physical, chemical and spectral methods. A tentative mechanism has been proposed with overall reaction.