In 2004, we started to develop a group contribution method aimed at estimating the temperature dependent binary interaction parameters (k(ij)(T)) for the widely used Peng-Robinson equation of state (EOS). This model was called PPR78 (predictive 1978, Peng-Robinson EOS) because it relies on the Peng-Robinson EOS as published by Peng and Robinson in 1978 and because the addition of a group contribution method to estimate the k(ij) makes it predictive. In our previous papers, 14 groups were defined: CH3, CH2, CH, C, CH4 (methane), C2H6 (ethane), CHaro, C-aro, C-fused (aromatic) (rings), CH2.cyclic, CHcyclic=C-cyclic, CO2, N-2, and H2S. It was thus possible to estimate the k(ij) for any mixture containing alkanes, aromatics, naphthenes, CO2, N-2, and H2S whatever the temperature. In this study, the PPR78 model is extended to systems containing thiols (also called mercaptans). To do so, the sulfhydryl group: -SH was added. (C) 2008 Elsevier Ltd. All rights reserved.