The collective variables method with a reference system is used for the investigation of phase transitions in the binary mixture. The explicit expressions for critical temperatures of a two-component system of different size particles interacting via different attractive potentials are obtained in the Gaussian approximation. The phase space of collective variables connected with order parameters is found. The case of the symmetrical mixture is considered in detail. The results are compared with the already known ones obtained for hard-core Yukawa mixture by the integral equation methods. The main difference is a presence of the second branch of critical temperatures. Numerical results are in a good agreement with those obtained in the mean-spherical approximation.