The photochemistry of aqueous solutions of the cis,trans-[PtIV(en)(I)2(CH3COO)2] complex (1) was studied by stationary photolysis, nanosecond laser flash photolysis and ultrafast kinetic spectroscopy in the time interval from hundreds of femtoseconds to minutes. It is shown that the multistage photolysis, including photochemical reactions of ligand exchange and photoreduction, involves in the fi rst step the formation of the cis-[IV(en)(I)2(CH3COO)2] complex with a quantum yield of 0.3 (irradiation at 282 nm). The further ligand exchange and photoreduction reactions occur with much lower quantum yields. The photochemical exchange of the acetate ion occurs in a time interval of ~100 ps with the much slower final step, being probably the acidic dissociation of the product. The key intermediate of the first step of the process was detected in the picosecond time range. Depending on the interpretation of the key intermediate nature, two possible mechanisms of the primary photochemical process are proposed.