The activity coefficient at infinite dilution (limiting activity coefficient) represents an important property, which is used in particular for the selection of selective solvents (e.g. extraction and extractive distillation) and for the reliable design of thermal separation processes. The removal of the last traces of impurities causes the largest separation effort. In order to avoid an oversizing of the column (e. g. distillation column) which would lead to an increase of the investment and operating costs, reliable information about the separation factor at infinite dilution is required.