  The microscopic approach to the description of the phase behaviour and
critical phenomena in binary fluid mixtures is proposed. It is based on the
method of collective variables with a reference system. The physical nature of
the order parameter in a binary mixture is discussed. The basic density measure
(Ginsburg-Landau-Wilson Hamiltonian) is obtained in the collective variable
phase space which contains the variable connected with the order parameter of
the system. It is shown that the problem can be reduced to the 3D Ising model
in an external field.
