The trinuclear complex (NH3)5CoIII(μ-NC)CoIII(CN)4(μ-CN)RuII(CN)53− shows a long wavelength absorption near 370 nm which is assigned to a metal-to-metal charge transfer (MMCT) transition from Ru(II) to the central Co(III) ion. Light absorption by this bands leads to a redox decomposition: (NH3)5CoIII(CN)4(μ-CN)RuII(CN)53−→5NH3+ Co2++CoIII(CN)63−+ RuIII(CN)5H2O2−. It is suggested that the initial MMCT transition is followed by a charge shift from the central Co(II) to the cobalt(III) ammine moiety which subsequently undergoes a rapid irreversible decay.