Group-contribution-based property estimation methods are suitable for obtaining quick evaluations of phase equilibrium under different conditions of temperature, pressure and composition. One of the best known and successful group-contribution (GC) based methods for prediction of liquid-phase-activity coefficients in mixtures of organic compounds is the UNIFAC method. One of the principal limitation of the UNIFAC method and all other GC based methods is that groups or group-interaction parameters needed for a specific property estimation problem may not be available. The method presented, is to be called the CI-UNIFAC (Connectivity Index - UNIFAC) method, to predict the missing UNIFAC group-interaction parameters for the calculation of vapor-liquid equilibrium (VLE). The method is described as the CI-UNIFAC for predicting missing group interaction parameters, as well as re-estimating known group interaction parameters, using a set of atom connectivities and their regressed CI interaction parameters. The performance of the CI-UNIFAC method with experimental data is compared, with a reference UNIFAC method, as well as cases where the CI-UNIFAC method is used only for the missing UNIFAC group interaction parameters. (c) 2007 American Institute of Chemical Engineers