The maximum likelihood method was applied to the calculation of parameters of a thermodynamic model in a heterogeneous binary system. The calculation program was tested on using the tetrahydrofuran-water system.When correlating the mutual solubility of liquids, three procedures are in principle used. In the first case, the values of composition x I and Z I are directly correlated as a function of temperature -see, e.g., Tsonopoulos and co-workers 1 • 2 • In the second case, the relations are applied which stem from the new knowledge of the behaviour at the critical point and in its vicinity -see, e.g., Vnuk 3 • Most often is used, however, the third (thermodynamic) procedure in which one starts from the equation for the excess Gibbs energy GE(T, Xl) or from equations of state.In this work we apply the thermodynamic procedure and parameters {b j } in the relation for the excess Gibbs energy are determined above all on the basis of data on mutual solubility (thereinafter LLE data). The advantage of this approach is above all the fact that the correlation obtained is not being an end in itself or it has not only the interpolation purpose but is utilizable also for the prediction of behaviour of multicomponent systems. This procedure has been used by a number of authors 4 -1o .When determining the parameters, the classical least-squares method has recently been replaced by the maximum likelihood method 11 -13 (thereinafter ML method) which takes fully into account all the experimental facts and is applied also in this work.