The photochemical reaction of acidic solution of carboplatin upon 313 nm irradiation has been studied by HPLC method.It it found that the photolytical product is cis-[Pt(NH3)2·(H2O)2]2+ and the quantum yield is 0.89.The result shows that acidity plays a key role in the photoreaction of carboplatin.The mechanism of the action of acidity has also been discussed.