The use of two modified BET models for modeling water vapor sorption in various nonideal systems involving strong interactions (textiles, polyamides, polyimide, vinyl polymers, proteins, and related compounds) has been investigated. Contrary to the original BET model derived for multimolecular adsorption and restricted to low water activities only, the three-parameter BET model, which considers sorption on a limited number of sorption layers, allowed the modeling for activities up to 0.9. This model has been shown to be an important extension of the original model but remains limited, due to the divergence of the model for a --> 1. Releasing an other important assumption of the BET model, the GAB model was then investigated and its striking efficiency, in terms of high correlation coefficients and low residual sums of squares, was demonstrated for activities up to 0.95, thus covering all the usual experimental range.