The rate constant ( k(H)) of hydride transfer from an NADH analogue, 9,10-dihydro-10-methylacridine ( AcrH(2)), to 1-(p-tolylsulfinyl)-2,5-benzoquinone ( TolSQ) increases with increasing Sc3+ concentration ([ Sc3+]) to reach a constant value, when all TolSQ molecules form the TolSQ-Sc3+ complex. When AcrH(2) is replaced by the dideuterated compound ( AcrD(2)), however, the rate constant ( k(D)) increases linearly with an increase in [ Sc3+] without exhibiting a saturation behavior. In such a case, the primary kinetic deuterium isotope effect ( k(H)/k(D)) decreases with increasing [ Sc3+]. On the other hand, the rate constant of Sc3+-promoted electron transfer from tris( 2-phenylpyridine) iridium [ Ir( ppy)(3)] to TolSQ also increases linearly with increasing [ Sc3+] at high concentrations of Sc3+ due to formation of a 1: 2 complex between TolSQ(.-) and Sc3+, [ TolSQ(.-)( Sc3+)(2)], which was detected by ESR. The significant difference with regard to dependence of the rate constant of hydride transfer on [ Sc3+] between AcrH(2) and AcrD(2) in comparison with that of Sc3+-promoted electron transfer indicates that the reaction pathway is changed from one-step hydride transfer from AcrH(2) to the TolSQ-Sc3+ complex to Sc3+-promoted electron transfer from AcrD(2) to the TolSQ-Sc3+ complex, followed by proton and electron transfer. Such a change between two reaction pathways, which are employed simultaneously, is also observed by simple changes of temperature and concentration of Sc3+.