A new concept named mass connectivity index (MCI) to encode bond contributions and to allow quantifying the extent of branching in a molecule, especially of ionic liquids, is proposed. The concept is based on the molecular connectivity concept first introduced by Randic in 1975 and modified in different forms by several authors. The proposed concept is much easier to calculate than any of the connectivity indexes available in the literature. Thus the only data required for determining the MCI are the groups forming the molecule and the mass of the groups. The groups forming the molecule are defined as commonly done in group contribution methods. The index is used as a variable in generalized correlations to accurately estimate the density and the heat capacity of ionic liquids. (C) 2010 Elsevier B.V. All rights reserved.