Sodium nitroprusside contains cyanide ligands that cannot be so easily replaced chemically. An effort has been made to substitute this ligand by another ligand like dimethylurea. The progress of this photochemical reaction has been observed spectrophotometrically and the product was isolated and characterized. The effect of different parameters like pH, intensity of light concentration of sodium nitroprusside and ligand concentration on the yield of this substitution reaction has been studied. A tentative mechanism for this reaction has been proposed.