The reaction of ethylene glycol dimercaptoacetate and pentaerythritol tetramercaptoacetate withp-benzoquinone have been investigated, both in solution and in the presence of cellulose fibers. In either system, Michael addition is the predominant reaction, leading to decolorization of the quinone by converting it to a substituted hydroquinone. This reaction accounts for the initial bleaching of high-yield pulp produced by these compounds. Handsheets impregnated with these compounds have been irradiated for extended periods of time, and are found ultimately to become more discolored than untreated handsheets. The darkening may be the result of photoinitiated redox chemistry between the Michael adducts and quinones produced in the photodegradation of lignin.