It is shown that cubic equations of state of the van der Waals family predict the ideal behavior of athermal solutions at infinite pressure. An expression for the infinite-dilution activity coefficient in binary mixtures is derived from the Soave-Redlich-Kwong (SRK) equation of state. The nonresidual contribution of this expression is used to predict the activity coefficients of athermal mixtures. In this work, we compare experimental infinite-dilution activity coefficients with predictions from different models. The comparison shows better agreement of the present model for light linear and branched alkanes diluted in normal alkanes of 20-36 carbon atoms.