A new UNIFAC (extended model), to be called KT-UNIFAC, is proposed for estimation of mixture properties (activity coefficients and excess enthalpies) for vapor-liquid equilibrium from group contributions. Estimation is performed at two levels: the basic level uses contributions from first-order simple groups, while the second level uses a small set of second-order groups having the first-order groups as building blocks. The role of the second-order groups is to consider, to some extent, the proximity effects and to distinguish among isomers. In the second-order UNIFAC model (Fluid Phase Equilib. 1999, 158, 349), the excess Gibbs function is calculated as a sum of a first-order combinatorial contribution, a first-order residual contribution, and a second-order residual contribution. The sets of first- and second-order groups have been revised and extended. The performance of this KT-UNIFAC model has been tested through correlation and prediction of vapor-liquid equilibria, infinite dilution activity coefficients, and excess enthalpies covering data involving 4413 binary mixtures and 27 ternary systems. Compared with some of the currently used versions of UNIFAC, the KT-UNIFAC model makes significant improvements in accuracy while providing a much wider range of applicability.