Ahstract The kinetics of thermal reactions of photochemically generated [Mo(CN),(CNH)(H,O)]'with l,lO-phenanthroline and 2,2'-bipyridyl in acidic medium were studied in buffer solutions of pH 4.2-7-O and ionic strength 8.0X 10e2 M at 25 "C. The quantum yield of formation of the photoproduct was calculated and was found to depend on the pH, ligand and [Mo(CN),14-concentration. A reaction scheme for the photochemical and thermal reactions of octacyanomolybdate(IV) ion with l,lO-phenanthroline and 2,2'-bipyridyl is proposed. The rate of the reaction and the quantum yield increase with an increase in hydrogen ion and ligand concentration and decrease with an increase in [Mo(CN),14-concentration. In acidic medium the excited species, [Mo(CN),14-*, protonated at the cyano group which causes the electron density to shift from the Mo-CN bond axis towards the cyano group. On protonation the v and CZY bonds of Mo-CN behave oppositely, i.e. the former becomes stronger and the latter becomes weaker leading to elongation and consequent weakening of the Mo-CN bond. Therefore the rate of reaction and the quantum yield increase at lower pH.