This work aimed at extending the group interaction matrix of the NIST-UNIFAC method to allow the prediction of activity coefficients of compounds in mixtures containing lactones. New 46 parameter values for 12 main groups were regressed based on VLE and LLE data from the literature (654 data of 51 mixtures). They comprised the interactions between the representative main groups of lactone and alkane (CH2), aromatic (ACH and ACCH2), alcohol (OH), water (H2O), ketone (CH2CO), aldehyde (CHO), ester (CCOO), ether (CH2O), carboxylic acid (COOH), cyclic alkane (c-CH2), and tetrahydrofuran (c-CH2O). The parametrization resulted in very satisfactory predictions, especially for the VLE mixtures. The overall root mean square deviation for LLE systems was SRMSD = 3.24%, and the best and worst performances occurred for alcohol (SRMSD = 1.05%) and aromatics (SRMSD = 4.68%), respectively. For the VLE, the overall deviation of vapor phase was SRMSD = 2.04%. The best results were attained for systems with alcohol (SRMSD = 0.10%), while those mixtures containing ketones resulted in the highest deviation (SRMSD = 3.59%).