The critical phenomenon of nonaqueous microemulsion was studied for the first time. The coexistence curves of (T, n) (T, phi), and (T. psi) (n and psi are refractive index and volume fraction, respectively; psi is defined as psi = phi/[phi+phi(c)(1-phi)/(1-phi(c))]) for a tertiary microemulsion [phi (AOT-DMA) + (1-phi) decane] at constant pressure and a constant molar ratio (omega = 2.86) of DMA to AOT have been determined within about 7 K from the critical temperature T-c by measurements of refractive index. The critical exponent has been deduced from (T. 11), (T. phi), and (T, psi) coexistence curves within 1 K below T-c. They all were 0.329 +/- 0.005 and were consistent with the 3D Ising value. The experiniental results in a temperature range of (T-c - T) < 7 K also have been analyzed to obtain critical amplitudes and the Wegner correction terms. to examine the diameters of the coexistence curves. (c) 2005 Elsevier Inc. All rights reserved.