The coexistence curves of a ternary microemulsion system of {water+sodium di(2-ethylhexyl) sulfosuccinate (AOT) + n-decane} with the molar ratio (30.0) of water to AOT have been determined by measurements of refractive index at constant pressure within about 8 K from the critical temperature T-c. The critical exponent beta and the critical amplitude B have been deduced from the coexistence curves. The experimental results have been analysed and compared with the system with molar ratio of 40.8 studied previously. It was found that the critical exponents beta for both systems were consistent with the 3D-Ising value in a region sufficiently close to the critical temperature. The critical concentration was slightly affected by the molar ratio omega, but the critical temperature significantly was raised as the molar ratio omega was decreased. The volume fraction phi was the better choice of the concentration variable than the effective volume fraction psi and the refractive index n used for constructing the order parameter for both systems. (c) 2007 Elsevier Ltd. All rights reserved.