A three-component microemulsion system can be regarded as a pseudobinary mixture at certain conditions. The coexistence curves of (T, n), (T, phi), and (T, Psi) (n and phi are refractive index and volume fraction, respectively; Psi is defined as Psi = phi/[phi+phi(c)(1-phi)/(1-phi(c))]) for a ternary microemulsion (AOT-water-decane) at constant pressure and a constant ratio of water to AOT have been determined within about 10 K from the critical temperature by measurements of refractive index. The critical exponent beta was deduced precisely from (T, n), (T, phi), and (T, Psi) coexistence curves within 1.5 K above T-c. These values were 0.327, 0.324, and 0.325, respectively, and were consistent with the 3D Ising exponent. The volume fraction phi is a better choice of the concentration variable than Psi used for construction of the order parameter to fit the Ising behavior.