The study of phase equilibria in hydrogen-containing mixtures is essential for petroleum and chemical engineering, electricity production, transportation and for many other energy needs. Fluid-phase diagrams are however atypical because of the size-asymmetric nature of these mixtures and the quantum behavior of hydrogen. Therefore, the development of a thermodynamic model able to accurately predict the phase behavior of such systems over wide ranges of pressure and temperature is a difficult and challenging task. In this work, the H-2 group is added to the well-established PPR78 model in order to predict mutual solubility and critical lad of hydrogen-containing systems. Such a model combines the widely used Peng-Robinson equation of state (EoS) with a group-contribution method aimed at estimating the temperature-dependent binary interaction parameters [k(ij)(T)]. In our previous papers, 15 groups were defined: CH3, CH2, CH, C, CH4 (methane), C2H6 (ethane), CHaro, C-aro, C-fused (aromatic rings), CH2,cyclic, CHcyclic double left right arrow C-cyclic, CO2, N-2, H2S, and -SH. It was thus possible to estimate the k(ij) for any mixture containing alkanes, aromatics, naphthenes, CO2, N-2, H2S and mercaptans regardless of the temperature. In this study, the addition of the H-2 group makes it possible to extend the PPR78 model to hydrogen-containing systems. (c) 2012 Elsevier B.V. All rights reserved.