In this study the use of sodium nitroprusside as nitric oxide donor in solutions was utilized. It has been established that maximum release of nitric oxide is achieved under irradiation by UV light at 254nm. The synergistic effects of cobalt trimethylene diamine and nitroprusside towards the hydrolysis of nitrophenylphosphate under the above conditions for different cobalt to nitrophenylphosphate ratio were investigated. This study demonstrates that besides the effect of hydroxyl radicals, the direct interaction of nitric oxide with the phosphorous center also play a role in decontamination reactions of poorly biodegradable phosphate esters in natural waters, due to phototransformation.