The anisotropic swelling of Nafion 112 membrane in pure organic liquids was monitored by an optical method. The findings were used as a basis for application of the group contribution method to the relative expansion in equilibrium. From a total of 38 organic liquids under study, 26 were selected as an evaluational set from which the group and structural group contributions were assigned. The remaining 12 compounds were used as the testing set. Value of +/-1.5% in relative expansions was determined to be the experimental error. Maximum differences between the experimental and calculated relative expansions in both sets did not exceed the value of +/-3%. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1745-1750, 2009