Two rigid bischromophoric systems featuring identical donor and acceptor moieties have been studied in order to directly determine the relative importance of through-space and through-bond mechanisms for electron-transfer reactions. The two molecular systems studied have the unique feature that the through-bond distance between any pair of atoms in the two molecules is held constant while the spatial distance between the donor and acceptor is changed. Time-resolved laser measurements reveal that back-electron-transfer dynamics following photoexcitation of the ground-state charge-transfer absorption band are the same in the two systems. The results provide direct evidence for the through-bond mechanism of electron transfer in bridged organic donor-acceptor systems.