The coexistence curves of {water + KCl + sodium di(2-ethyl-1-hexyl)sulphosuccinate (AOT) + decane} microemulsion system have been determined by measuring refractive index at constant pressure in the critical region. The critical exponent beta has been deduced from coexistence curves. The values of beta decrease with increase in the temperature range (T - T-c) and approach the Ising value in the region of 1 K above T-c. The experimental results have been analyzed to examine the Wegner correction terms and the behaviors of the diameters of the coexistence curves. It was found that addition of salt KCl into the ternary microemulsion of {water + AOT + decane} almost did not change the critical concentration but significantly raised the critical temperature. Addition of salt also significantly reduced the region of validity of simple scaling and the efficiency of the variable of volume fraction phi as an order parameter to describe the critical asymptotic behaviors. (C) 2004 Elsevier Ltd. All rights reserved.