The necessity of using a pressure (P) or temperature (T) dependence in the Gibbs energy of mixing of the liquid phase (g(M.L)) for some isothermal (or isobaric) VLE data sets, respectively, is discussed in this paper. A graphical representation, directly obtained from the experimental data, is proposed as a method to clearly classify the behaviour of the systems and select the adequate model (i.e. including when appropriate the T or P dependence) to correlate them. The result is that many of the poorly fitted systems in the literature could be precisely modelled using the correct procedure with adequate functions of P or T. Some suitable examples are used to illustrate the validity of these ideas, providing satisfactory correlation results for those systems in the literature. The ideas presented in this paper reveal important aspects related to the inappropriate application of some thermodynamic consistency (TC) tests to evaluate the quality of VLE data sets. (C) 2019 Elsevier B.V. All rights reserved.