This paper presents an approach to extend the applicability of the Group Contribution Solvation model (GCS) model to predict infinite dilution activity coefficients, gamma(infinity), for organic compounds in water. The use of a higher quantum level as proposed here does not eliminate the need of a parameterisation of the continuum solvation model used to determine the interaction parameters in the GCS model. The size of the solute cavities used in the two solvation calculations involved need to be treated differently. A procedure to optimise the size of the cavities used in water solvation calculations, by optimising the values of alpha-group scaling factors, is done based on a limited amount of experimental data on gamma(infinity). A better expression for the combinatorial contribution to gamma(infinity) is used in the GCS model. The results of a limited number of quantum calculations for medium sized solutes are further used to extend the applicability of the model for compounds within a homologous series. The estimated gamma(infinity) for the series of n-alkanes, n-alkanols and methyl-ketones deviate with less than 0.5 logarithmic units from the experimental values, and are more accurate than the ones predicted by the widely used MUNIFAC model or by the newly developed COSMO-RS and COSMO-SAC models. (C) 2004 Elsevier B.V. All rights reserved.