Photogenerated carrier separation is important in photocatalysis. Doping may offer control over the effective masses of the photogenerated electrons and holes. Herein, a doping strategy in Li 2 SnO 3 enhanced photogenerated carrier separation, boosting photocatalysis. Substitution of Ge with Sn increased the effective mass of holes and reduced that of electrons; hence, the photogenerated electron/hole lifetime ratio in Li 2 Sn 0.90 Ge 0.10 O 3 was approximately 2.8 times as great as that of Li 2 SnO 3 . Photocatalytic degradation by Li 2 Sn 0.90 Ge 0.10 O 3 reached 100% within 12 min. However, the opposite effect was achieved upon doping with Pb. Theoretical calculations revealed that the low Ge-4p valence band orbital reduced hole mobility, while the Ge-4s orbital hybridized with O-2p near the conduction band minimum increased the electron mobility. Steady-state and time-resolved photoluminescence spectroscopy, electron spin resonance, and liquid chromatography−mass spectrometry were conducted to explore the photocatalytic mechanism. This study provides an understanding of structure−activity relationships to guide the design of high-performance photocatalysts.