Out-of-home leisure activities are often conducted jointly by individuals implying that location and travel choices made for these activities are the result of a group interaction. Current utility-theoretic approaches assume an aggregated group utility function and hence ignore aspects of the group decision making process. In this study, an empirical model of joint-activity choice is developed that, in contrast, assumes a negotiation process. A social utility function describes how individuals deal with preference differences in the group. The model is estimated based on an experimental activity-travel choice task where group settings are mimicked. A sample (N = 315) from a national panel of individuals participated in the experiment. Estim...