The coexistence curves of (T, n), (T, phi) and (T, psi) (n, phi, psi are the refractive index, the volume fraction. and the effective volume fraction, respectively) for two ternary nonaqueous microemulsion systems have been determined at constant pressure and a constant molar ratio omega of DMA to AOT. The critical exponent beta and the critical amplitude B have been deduced, and the values of beta were consistent with the theory of 3D-Ising in a region sufficiently close to the critical temperature. The results show that the coexistence curves have been successfully described by the Wegner equation and the expression for the diameter. (c) 2005 Elsevier Ltd. All rights reserved.