We have described the behavior of a subgroup of regular solutions not very far from ideality (Hildebrand, 1929, 1947, 1951) that is analogous to the behavior of ideal solutions. For this subgroup the composition of each phase in equilibrium may be related logarithmically. We have found that some cases of isothermal, isobaric and ternary liquid-vapor systems in equilibrium follow this scheme. In each case the resulting calculations of phase compositions are in good agreement with the experimental data reported in the literature. This approach is consistent with correlations frequently used in liquid-liquid case (known as the Othmer-Tobias (1942) and Hand (1930) equations), as well as adsorption of liquid on solid (known as Langmuir-Freundlich isotherm (Dabrowski et al., 1979)). For fluids mixtures presenting this behavior we use the term of semi-regular mixtures. For this kind of fluid mixtures it can be found a simplified procedure to relate the equilibrium compositions that may be used as "shortcut" for chemical engineering designs. (C) 2011 Elsevier Ltd. All rights reserved.