A simple method is developed to estimate mixture critical temperatures (T-c), pressures (P-c), and densities (rho(c)) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO2-C3H8, CO2-nC(4)H(10), C2H6-C3H8, and C3H8-nC(4)H(10). Isothermal Liquid-vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (rho(l) and rho(v)), and coexisting compositions for the more volatile component (x(1)(v) and x(1)(l)) are used. In the first step, the difference of the saturated liquid and vapor densities (rho(l) - rho(v)) is fitted to an empirical function in ((P-c - P)/P-c) to obtain P-c. Then P/P-c and ((rho(l) + rho(v))/2 rho(c)) are simultaneously fitted to functions of a points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO2-C3H8 and CO2-nC(4)H(10), for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C2H6-C3H8 and C3H8-nC(4)H(10), for which there are sparse data. For all four mixtures, the critical temperature line, T-c vs. X-1, matches literature values within +/- 0.5%. The critical pressure Line, P-c vs. X-1, and critical density line, rho(c) vs. X-1, match literature values, in general, within +/- 2%.