One of the steps of a common pathway for biological energy conversion involves electron transfer between cytochrome c and cytochrome bc1. To clarify the mechanism of this reaction, we examined the structural association of those two proteins using the electron transfer-independent electron paramagnetic resonance (EPR) techniques. Drawing on the differences in the continuous wave EPR spectra and saturation recoveries of spin-labeled bacterial and mitochondrial cytochromes c recorded in the absence and presence of bacterial cytochrome bc1, we have exposed a time scale of dynamic equilibrium between the bound and the free state of cytochrome c at various ionic strengths. Our data show a successive decrease of the bound cytochrome c fraction as the ionic strength increases, with a limit of approximately 120 mm NaCl above which essentially no bound cytochrome c can be detected by EPR. This limit does not apply to all of the interactions of cytochrome c with cytochrome bc1 because the cytochrome bc1 enzymatic activity remained high over a much wider range of ionic strengths. We concluded that EPR monitors just the tightly bound state of the association and that an averaged lifetime of this state decreases from over 100 micros at low ionic strength to less than 400 ns at an ionic strength above 120 mm. This suggests that at physiological ionic strength, the tightly bound complex on average lasts less than the time needed for a single electron exchange between hemes c and c1, indicating that productive electron transfer requires several collisions of the two molecules. This is consistent with an early idea of diffusion-coupled reactions that link the soluble electron carriers with the membranous complexes, which, we believe, provides a robust means of regulating electron flow through these complexes.