To examine the critical behavior of the microemulsion. we have determined the coexistence curves for two tertiary microemulsion systems of {water + sodium di(2-ethylhexyl) sulfosuccinate (AOT) + n-decane} with the molar ratios omega = (45.2 and 50.0) of water to AOT, respectively, by measuring refractive index at a constant pressure in the critical region. The critical exponent beta and the critical amplitude B have been deduced from the coexistence curves. It was found that the values of beta for both systems were consistent with the 3D-Ising exponent in a critical region. By increasing omega, i.e. the droplet size, the critical temperature and, to a lesser extent, the critical concentration decrease. The region of coexisting two phases was drastically reduced by an increase in the droplet size. (C) 2009 Elsevier Ltd. All rights reserved.