Despite the advances in group decision support system (GDSS) research, few GDSS studies concentrate on problem-modeling tools to support decisions that cross boundaries of functional areas within the business. These decisions have a substantial effect on the profitability of the firm and account for much time and effort of senior management. This research investigates the effect of problem structuring and modeling with a GDSS on coordinated decision-making of managers in a group faced with a mixed-motive production-planning task. In a laboratory experiment, a GDSS with features supporting problem modeling is contrasted with a GDSS without such features. The results indicate that the groups using a GDSS with a problem-modeling tool outperformed the groups using a GDSS without a problem-modeling tool, but they were less efficient with respect to the time and number of messages it takes the group to converge to a final solution. User confidence in the solution did not differ between the two groups. The results of this study indicate that the problem modeling feature of a GDSS significantly influences group decision process and outcomes.