Medium-chain acyl-CoA dehydrogenase reduced with octanoyl-CoA is reoxidized in two one-electron steps by two molecules of the physiological oxidant, electron transferring flavoprotein (ETF). The organometallic oxidant ferricenium hexafluorophosphate (Fc+PF6-) is an excellent alternative oxidant of the dehydrogenase and mimics a number of the features shown by ETF. Reoxidation of octanoyl-CoA-reduced enzyme (200 microM Fc+PF6- in 100 mM Hepes buffer, pH 7.6, 1 degree C) occurs in two one-electron steps with pseudo-first-order rate constants of 40 s-1 and about 200 s-1 for k1 and k2, respectively. The reaction is comparatively insensitive to ionic strength, and evidence of rate saturation is encountered at high ferricenium ion concentration. As observed with ETF, the free two-electron-reduced dehydrogenase is a much poorer kinetic reductant of Fc+PF6-, with rate constants of 3 s-1 and 0.3 s-1 (for k1 and k2, respectively) using 200 microM Fc+PF6-. In addition to the enoyl-CoA product formed during the dehydrogenation of octanoyl-CoA, binding a number of redox-inert acyl-CoA analogues (notably 3-thia- and 3-oxaoctanoyl-CoA) significantly accelerates electron transfer from the dehydrogenase to Fc+PF6-. Those ligands most effective at accelerating electron transfer favor deprotonation of reduced flavin species in the acyl-CoA dehydrogenase. Thus this rate enhancement may reflect the anticipated kinetic superiority of anionic flavin forms as reductants in outer-sphere electron-transfer processes. Evidence consistent with the presence of two distinct loci for redox communication with the bound flavin in the acyl-CoA dehydrogenase is presented.