The complexes trans,cis- and cis,cis-[Ru(I)(Me)(CO) 2 (α-diimine)] and trans,cis-[Ru(I)(Bz)(CO) 2 (α-diimine)] (α-diimine 4,4'-dimethyl-2,2'-bipyridine (bpy'), N,N'-di-isopropyl-1,4-diazabutadiene (iPr-DAB)) are the subject of a photochemical study. Replacement of R Me by R Bz results in a dramatic change in the photochemical behaviour of the trans,cis-complexes. A photoisomerisation is observed for the complex trans,cis-[Ru(I)(Me)(CO) 2 (bpy')], whereas irradiation of trans,cis-[Ru(I)(Bz)(CO) 2 )(α-diimine)] results in the formation of [Ru(I) 2 (CO) 2 )(α-diimine)]. According to the nanosecond transient absorption spectra, the primary photoprocess of both trans,cis- and cis,cis-[Ru(I)(Me)(CO) 2 (bpy')] is loss of CO, whereas the corresponding complexes trans,cis-[Ru(I)(Bz)(CO) 2 (α-diimine)] (α-diimine bpy' and iPr-DAB) undergo homolysis of the Ru-Bz bond. This dependence of the primary photoprocess on R is ascribed to a crossing to different reactive excited states after occupation of a nonreactive 1 XLCT state. For R Me this state is proposed to have LF character, for R Bz it is most probably the 3 σπ * state.