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). In this approach, the k(ij) between two components i and j is a function of temperature (T) and of the pure component critical temperatures (T-c.i and T-c.j), critical pressures (P-c.i and P-c.j) and acentric factors (omega(i), omega(j)). Because our model 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, this model was called PPR78 (predictive 1978, Peng-Robinson EOS). In our previous papers eight groups were defined: CH3, CH2, CH, C, CH4 (methane), C2H6 (ethane), CHaro, and C-aro. It was thus possible to estimate the k(ij) for any mixture containing alkanes and aromatics whatever the temperature. In this study, the PPR78 model is extended to systems containing naphtenic (cyclic hydrocarbons) compounds. To do so, two new groups were added: CH2,cyclic and CHcyclic = C-cyclic (c) 2006 Elsevier B.V. All rights reserved.