Interband optical transitions in CeSn₃ and LaSn₃ have been investigated by measuring and calculating from band theory the optical conductivities of both compounds. For both compounds, interband transitions start at low energies (0.3 eV for CeSn₃ and 0.5 eV for LaSn₃) and these structures are mostly due to d to f and f to d transitions between hand-like d and f states originating from Ce or La atoms. The structures at higher energies (greater than 1 eV) strongly involve f to d and d to p transitions with p states originating from Sn atoms. Good qualitative agreement between calculation and experiment has been achieved for these higher-energy structures for both compounds. The larger strength of the structures in CeSn₃ above 1 eV is mainly due to the existence of more occupied band-like f states in CeSn₃ than in LaSn₃.