Accurate measurements of water, oil, and surfactant self-diffusion coefficients have been made in the microemulsion phase of pentaethylene glycol dodecyl ether (C(12)E(5)), water, and decane using nuclear magnetic resonance (NMR) techniques. In conjunction with previously measured H-2 NMR relaxation rates, the self-diffusion coefficients are consistent with a limited growth from sphere to prolate (oil-swollen) micelles with an axial ratio less than or equal to 3.5 as a precursor to the bicontinuous structure.