The application of a commonly used computer algorithm based on the group-additivity method for the calculation of the liquid viscosity coefficient at 292.15K and the activity coefficient at infinite dilution in water at 298.15K of organic molecules is presented. The method is based on the complete breakdown of the molecules into their constituting atoms, further subdividing them by their immediate neighbourhood. A fast Gauss-Seidel fitting method using experimental data from literature is applied for the calculation of the atom groups’ contributions. Plausibility tests have been carried out on each of the calculations using a 10-fold cross-validation procedure which confirms the excellent predictive quality of the method. The goodness of fit (Q²) and the standard deviation (σ) of the cross-validation calculations for the viscosity coefficient, expressed as log(η), was 0.9728 and 0.11, respectively, for 413 test molecules, and for the activity coefficient log(γ)^∞ the corresponding values were 0.9736 and 0.31, respectively, for 621 test compounds. The present approach has proven its versatility in that it enabled at once the evaluation of the liquid viscosity of normal organic compounds as well as of ionic liquids.