Two UNIFAC association models are developed using Wertheim’s theory of association rather than a chemical theory. In both models the activity coefficient is the sum of combinatorial, residual, and association contributions. The UNIFAC group-contribution model is used for the combinatorial and residual terms, and two different types of association models are considered. The UNIFAC-AG model uses functional-group-based association, while the UNIFAC-AM model considers association to occur between molecules. For associating mixtures containing acids, alcohols, or water, both activity coefficient models provide better predictions of binary vapor-liquid equilibria than the original UNIFAC model. Of those models, the UNIFAC-AM model led to the best predictions. The association term was also added to the more recent, modified UNIFAC model and briefly tested with vapor-liquid equilibrium data for the acetic acid + heptane and acetic acid + butanol mixtures. It was found that, for vapor-liquid equilibria, the modified UNIFAC + association model is only slightly better than the UNIFAC-AM model. However, the modified UNIFAC + association model has four more adjustable parameters than the UNIFAC-AM model. Therefore, use of the UNIFAC-AG or UNIFAC-AM models is recommended.