Binary systems of ammonia with impurities have been studied experimentally to obtain activity coefficients at 293.15K for liquid impurity concentrations from 0.6 to 1613 ppm. Activity coefficients for carbon monoxide, nitrogen and oxygen in ammonia increase with increasing concentration, in contrast to their decreasing in hydrocarbons. This behavior is consistent with molecular association. (C) 2015 Published by Elsevier B.V.