The kinetics of the photochemical reduction of molybdic acid in presence of d-tartarid acid have been studied and the quantum efficiency of the process has been measured. The dependence of the rate of reduction on the concentrations of both complex and uncombined molybdic acid, its variation with change of PH of the system and the appearance of different shades of the reduced solutions, have been explained. Evidence from rotation measurements of the reduced solutions indicates. that, the complex does not break into its components by reduction.