Studies on the electron transfer (ET) interaction of 1,4- dihydroxy-9,lO-anthraquinone and 6,1l-dihydroxy-5,l2-naph-thacenequinone with aliphatic and aromatic amine (AIA and ArA, respectively) donors have been investigated in acetonitrile solutions. Steady-state (SS) measurements show quenching of the quinone fluorescence by amines, without indicating any change in the shape of the fluorescence spectra. No significant change in the absorption spectra of the quinones is also observed in the presence of the amines. For all the quinone- amine pairs, the bimolecular quenching constants (k_q,) estimated from SS and time-resolved measurements are found to be similar. Variation in the k_q, values with the oxidation potentials of the amines indicates the involvement of the ET mechanism for the quenching process. A reasonably good correlation between the k_q, values and the free energy changes (∆G^O) for the ET reactions following Marcus' outer-sphere ET theory also supports this mechanism. It is seen that for both the quinone- ArA and quinone-AIA systems, the k_q, values initially increase and then get saturated at some diffusion-controlled limiting values (k_q^DC) as ∆G^O values gradually become more negative. Interestingly, however, it is seen that the k_q^DC value for the quinone-AIA systems is substantially lower than that for quinone-ArA systems. Such a large difference in the k_q^DC values between quinone-AIA and quinone-ArA systems is quite unusual. Present results have been rationalized based on the assumption that an orientational restriction is imposed for the  encounter complexes in quinone-AIA systems to undergo ET reactions, which arises because of the localized (at amino nitrogen) shapes of the highest-occupied molecular orbitals (HOMO) of AIA in comparison to the π-like HOMO of the ArA.