The photochemical reactions occurring in aqueous solutions of octacyanomolybdate(V) ion, Mo(CN)_8^3-, during irradiation with light of 365 and 436 nm wavelengths were studied. The quantum yields and reaction products were determined for neutral, unbuffered solutions as well as solutions buffered at pH 4.0, 7.0, and 9.0. The results of these studies showed that the Mo(CN)_8^3- ion is rapidly reduced by light to a Mo(IV) complex. In neutral or acidic media, the initial photoreduction product was found to be the red heptacyanoaquomolybdate(IV) ion, Mo(CN)_7(H_2O)^3-. Photolysis in basic solution gave the yellow heptacyanohydroxomolybdate(IV) ion, Mo(CN)_7(OH)^4-, as an unstable intermediate. Hydrolysis of this complex to the blue tetracyanohydroxooxomolybdate(IV) ion, Mo0(OH)(CN)_4^3-, occurred rapidly in base. In each case, cyanogen, (CN)_2, and cyanate ion, OCN^-, were formed. Additionally, in neutral solutions, the formation of hydrogen ion during the irradiation of the sample gave a rapid drop in the pH of the solution. Measurements of the product quantum yields for Mo(IV) complexes, *_Mo(IV), gave values that were always greater than one and which increased rapidly with increasing pH of the irradiated solutions, The values were the same for both irradiating wavelengths, showing no wavelength effect for the reaction, The quantum yields determined for *_Mo(IV) were 1.51, 1.59, 3.10, and 4.29 for pH 4 , neutral pH, pH 7, and pH 9 solutions respectively. From the experimental results, a reaction mechanism is suggested for each of the aqueous media used, The initial photolytic reactions are the same in all solutions with subsequent thermal reactions being responsible for the variation of *_Mo(IV) with the pH of the solution.