Electron transfer from reduced nicotinamide adenine dinucleotide (NADH) to the hydroxylase component (MMOH) of soluble methane monooxygenase (sMMO) primes its non-heme diiron centers for reaction with dioxygen to generate high-valent iron intermediates that convert methane to methanol. This intermolecular electron-transfer step is facilitated by a reductase (MMOR), which contains [2Fe-2S] and flavin adenine dinucleotide (FAD) prosthetic groups. To investigate interprotein electron transfer, chemically reduced MMOR was mixed rapidly with oxidized MMOH in a stopped-flow apparatus, and optical changes associated with reductase oxidation were recorded. The reaction proceeds via four discrete kinetic phases corresponding to the transfer of four electrons into the two dinuclear iron sites of MMOH. Pre-equilibrating the hydroxylase with sMMO auxiliary proteins MMOB or MMOD severely diminishes electron-transfer throughput from MMOR, primarily by shifting the bulk of electron transfer to the slowest pathway. The biphasic reactions for electron transfer to MMOH from several MMOR ferredoxin analogues are also inhibited by MMOB and MMOD. These results, in conjunction with the previous finding that MMOB enhances electron-transfer rates from MMOR to MMOH when preformed MMOR-MMOH-MMOB complexes are allowed to react with NADH [Gassner, G. T.; Lippard, S. J. Biochemistry 1999, 38, 12768-12785], suggest that isomerization of the initial ternary complex is required for maximal electron-transfer rates. To account for the slow electron transfer observed for the ternary precomplex in this work, a model is proposed in which conformational changes imparted to the hydroxylase by MMOR are retained throughout the catalytic cycle. Several electron-transfer schemes are discussed with emphasis on those that invoke multiple interconverting MMOH populations.