The diffusion processes underlying the observed transport properties of the polar solvent in some reverse aggregated, L(2) phase microemulsions has been studied. Three different surfactant-oil-polar solvent systems with the surfactants sodium bis(2-ethylhexyl) sulfosuccinate (AOT), didodecyldimethylammonium bromide (DDAB), and tetraethylene glycol dodecyl ether (C(12)E(4)), respectively, were-investigated. The polar solvent was dilute aqueous solutions of either N-methylformamide or tetramethylammonium chloride. These additives used have essentially no influence on the phase behavior compared to pure water. Following the approach of a previous publication (J. Phys. Chem. 1988, 92, 6675) we have measured the self-diffusion coefficients of the two polar species in the microemulsions, the ratio being sensitive to the nature of the diffusion process. In the AOT system a dominating aggregate diffusion process was found at lower temperatures indicating closed isolated reversed micellar aggregates. With increasing temperature, structural changes cause a crossover to a dominating molecular diffusion in a medium similar to the neat aqueous solution. This implies a crossover to a bicontinuous network at higher temperatures. In contrast, we found no region of dominating aggregate diffusion process in the DDAB and C(12)E(4) systems. A dominating or almost dominating molecular diffusion process was detected even when the absolute diffusion coefficients of the polar solvent species were low.