Electrochemical studies on oil-in-water vinyltoluene-sodium dodecyl sulfate (SDS) - water-n-pentanol microemulsions have been carried out to evaluate the microstructure and diffusion properties. Rotating disc voltammetry (RDV), electrical conductivity, and viscosity are measured at 27 degrees C to obtain the diffusion coefficients, droplet radii, surfactant aggregation number, fraction of free ions, number density of the droplets, etc. Ferrocene was used as the electrochemical probe. The droplet radii range from 25 to 75 Angstrom. The effects of compositional variations in vinyltoluene and SDS on the microstructure and diffusion properties are also discussed.