A new activity coefficient model is proposed base on the regular solution theory and composition dependent surface area parameters. Surface area parameters, which account for the number of interactions between molecules, may vary depending on partner molecules and the concentration of mixtures. Conventional models such as NRTL and UNIQUAC uses fixed number and they normally fail to describe vapor-liquid equilibria and liquid-liquid equilibria simultaneously. Composition-dependent surface area parameters were combined with interaction parameters to give four parameters for a binary mixture. The proposed model can successfully describe an exceptionally high value or convex behavior of activity coefficient in the dilute region. The proposed model also shows improved performances in correlating the liquid-liquid equilibria for ternary systems and the vapor-liquid equilibria for constituent binary systems with the same parameter sets. (C) 2018 Elsevier B.V. All rights reserved.