COSMO-based activity coefficient models, similarly to the UNIQUAC method, rely on two contributions: combinatorial and residual. The combinatorial term should account for differences in size, shape, and free volume. The residual term should account for electrostatics, dispersion, and hydrogen-bonding effects. The recent literature shows a great effort in improving the residual term, while less attention is given to the combinatorial contribution. This is partly justified by the fact that the residual contribution can be order: of magnitude larger than the combinatorial one. Nevertheless, once the activity coefficient of a substance in mixture is given by the product of these two contributions, both are important. In this work, the combinatorial expressions currently in use in COSMO-based models are reviewed. It is also shown that the model performance can be much improved by assuming a combinatorial contribution similar to that present in the modified UNIFAC model.