The photoreduction of aqueous sodium and potassium carbonates has been carried out in the presence of methylene blue solution (which is a photocatalyst also). The photocatalytic formation of formic acid and formaldehyde were measured spectrophotometrically using Nash reagent. The effect of variation of various parameters like pH, amount of photocatalyst (methylene blue concentration), concentration of Na2CO3 and K2CO3, light intensity, etc. on the yield of photoproducts was also investigated. A tentative mechanism for this reduction has been proposed.