A growing number of companies are outsourcing their purchasing processes to independent purchasing agencies. These agencies now have to process an ever increasing number of purchase requests each day. The conventional methods of selecting the right suppliers for the purchase requests incur heavy human and time costs. We have designed and implemented BestChoice, a decision support system for supplier selection. It allows the evaluator to create rules for supplier evaluation based on the Multi Attribute Utility Theory, a theory for evaluating the utility of alternatives. BestChoice provides rule structures that can be saved and reused for similar selection cases. The architecture and selection rules of BestChoice are presented. Performance of BestChoice at one of the largest procurement agencies is analyzed.