Tridentate Schiff base-ruthenium(II) complexes containing soft phosphorus or sulfur donor atoms were prepared, and their bidirectional thermal and photo substitution reactions were investigated. In acetonitrile, the chloro ligands of trans(Cl,Cl) [RuCl 2 (ppb-etol)(PPh 3 )] (1, ppb-etol=2-(Ph 2 P)C 6 H 4 CH N(CH 2 ) 2 OH) were substituted by acetonitrile molecules stepwise to yield [RuCl(CH 3 CN)(ppb-etol)(PPh 3 )]Cl (2) (first step) and [Ru(CH 3 CN) 2 (ppb-etol)(PPh 3 )]Cl 2 (3) (second step). Complexes 2 and 3 reverted to complex 1 in acetonitrile upon irradiation with UV-Vis light. Similar thermal and photo substitution reactions were also observed for trans(Cl,Cl) [RuCl 2 (btb-etol)(PPh 3 )] (4, btb-etol=2-( t BuS)C 6 H 4 CH N(CH 2 ) 2 OH). The substitution rates were measured for the first-step reactions of complexes 1 and 4 over the temperature range of 19.5-29.5 o C in acetonitrile. Kinetic parameters were determined to be k o b s d =(1.13+/-0.04)x10 - 4 s - 1 (25.0 o C), ΔH =83.1+/-1.7 kJ mol - 1 , and ΔS =-41.9+/-5.6 kJ mol - 1 for complex 1, and k o b s d =(1.57+/-0.06)x10 - 3 s - 1 (25.0 o C), ΔH =88.8+/-1.9 kJ mol - 1 , and ΔS =-0.9+/-6.5 kJ mol - 1 for complex 4. Accordingly, the substitution reaction of complex 4 proceeded faster than that of complex 1. The structures of [RuCl 2 {ppb-(1R,2S)-ephe}(PPh 3 )] (5, ppb-(1R,2S)-ephe=2-(Ph 2 P)C 6 H 4 CH NCH(Me)CH(Ph)OH), complex 2, and [Ru(CH 3 CN) 2 {ppb-(R)-btol}(PPh 3 )](BF 4 ) 2 (6, ppb-(R)-btol=2-(Ph 2 P)C 6 H 4 CH NCH(Et)CH 2 OH) were determined by X-ray analyses. In each complex, the geometry around the ruthenium atom is distorted octahedral, and the tridentate Schiff base and triphenylphosphine ligands occupy the equatorial positions.