The changes in viscosity exhibited upon mixing remain one of the major challenges for substance property prediction. Viscosity often drastically changes after even small changes in concentration of one of the compounds in the mixture. The viscosity behavior of mixtures of ionic liquids with water has been studied. First, a generalized evaluation of the shape of the viscosity-concentration curves has been done. About one-tenth of the investigated systems exhibited maxima or inflection points of the curve. The simple logarithmic mixing rule turns out to be applicable as a first approximation for many systems. However, it tends to underestimate mixture viscosity. Following these results, parameters for a group contribution method were fitted and evaluated. This method is based on the Grunberg and Nissan extension of the logarithmic mixing rule.