Electron-transfer rate constants were determined by means of lifetime measurements for the fluorescence quenching of 9,10-dicyanoanthracene by aromatic amines and methoxybenzenes as electron donors, and for the quenching of the synthetic dyes eosin Y and phenosafranine by a series of p-benzoquinones as electron acceptors. All determinations were carried out in acetonitrile at 298 K. The quenching rate constants (k(q)) in the region of -1.9 eV < DeltaG(et) < -0.2 eV do not decrease as predicted by Marcus theory, but they show a small increase with decreasing DeltaG(et). Although this behaviour is in qualitative agreement with the current theories for reactive systems in the diffusion limit region, a closer analysis of the experimental data showed that several aspects of the dependence of the k(q) on DeltaG(et) are not entirely explained, suggesting that new, refined theoretical models may be required.