Electron transfer rates were measured in RCs from three herbicide-resistant mutants with known amino acid changes to elucidate the structural requirements for last electron transfer. The three herbicide resistant mutants were IM(L229) (Ile-L229↦ Met), SP(L223) (Ser-L223↦ Pro) and YG(L222) (Tyr-L222↦ Gly). The electron transfer rate D(+)QA (-)QB↦D(+)QAQB (k AB) is slowed ∼3 fold in the IM(L229) and YG(L222) RCs (pH 8). The stabilization of D(+)QAQB (-) with respect to D(+)QAQB (-) (pH 8) was found to be eliminated in the IM(L229) mutant RCs (ΔG(0) ∼ 0 meV), was partially reduced in the SP(L223) mutant RCs (ΔG(0)=-30 meV), and was unaltered in the YG(L222) mutant RCs (ΔG(0)=-60 meV), compared to that observed in the native RCs (ΔG(0)=-60 meV). The pH dependences of the charge recombination rate D(+)QAQB (-)↦DQAQB (k BD) and the electron transfer from QA (-) (k QA (-)↦QA) suggest that the mutations do not affect the protonation state of Glu-L212 nor the electrostatic interactions of QB and QB (-) with Glu-L212. The binding affinities of UQ10 for the QB site were found in order of decreasing values to be native ≥IM(L229) > YG(L222)≥ SP(L223). The altered properties of the mutant RCs are used to deduce possible structural changes caused by the mutations and are dicscussed in terms of photosynthetic efficiency of the herbicide resistant strains.