Coexistence curves of (T, n), (T, phi), and (T, Psi), where n, phi and Psi are the refractive index, volume fraction and effective volume fraction psi = phi/{phi + [(1 - phi)phi(c)/(1 - phi(c))]}, respectively, for ternary microemulsion systems of (water + n-nonane + sodium di(2-ethyl-1-hexyl)sulphosuccinate} have been determined at temperatures within 8.7 K above the critical temperature by measurements of refractive index at constant pressure and a constant molar ratio of water to sodium di(2-ethyl-1-hexyl)sulphosuccinate. The critical exponent beta deduced from (T, n), (T, phi), and (T, Psi) coexistence curves was found consistent with nonmonotonic crossover observed in all aqueous ionic solutions. The values of deduced from the experimental data in the range of I K above T-c were consistent with the universality class of three-dimensional Ising-like systems. The coexistence curves have been interpreted by a combination of the Wegner expansion and the rectilinear diameter. The present results indicate that the molar mass dependence of critical amplitudes, we proposed recently, is valid for microemulsion systems.