The photophysical properties of bis-1,8-naphthalimide (NI-L-NI) dyads with different linkers (L = -C3H6-, -C4H8-, -C6H12-, -C8H16-, and -C9H18-) as well as the reference NI derivative (NI-C7H15) were investigated in CH3CN and H2O/CH3CN (v/v = 1:9). The normal fluorescence peak of (NI)-N-1*-L-NI was observed at 379 nm together with a broad emission at longer wavelength both in aprotic CH3CN and in H2O/CH3CN, which is assigned to an excimer, (1)(NI-L-NI)*. The excimer emission maximum was blue-shifted with increasing length of the linker. The photoinduced electron-transfer process of NI-L-NI was also investigated in both solvents by using nanosecond-laser flash photolysis. The T-1 -T-n absorption band for (NI center dot-)-N-3-L-NI was observed around 470 nm in both solvents. In H2O/CH3CN, NI-L-NI is solvated with H2O in the ground state to exist as solvated NI-L-NI. In the excited triplet state, the NI radical anion (NI center dot-) was generated via the intramolecular quenching of (NI*)-N-3-L-NI by another NI moiety. The solvated NI center dot-L-NI may undergo the proton abstraction process to give NI(H)-L-NI, which can be confirmed by the transient absorption band at 410 nm. This band was not observed in pure aprotic CH3CN.