In business markets, firms increasingly participate in new product development (NPD) activities that they outsource to suppliers. Such buyer participation can be complicated by relationship multiplexity-that is, the buyer may simultaneously be a competitor, supplier, and/or partner of the supplier firm. Drawing on role theory, we theorize how relationship multiplexity moderates the effect of buyer participation on project success. We analyze 140 NPD projects that were executed by a contract R&D organization in the aerospace industry and find that buyer participation lowers the buyer's and the supplier's perceptions of project success for buyer-as-supplier multiplexity. Buyer participation increases the perceptions of project success in the case of buyer-as-partner multiplexity, but only when these partnerships are emergent, as opposed to engineered. Buyer participation decreases the buyer's perception of project success for relationships featuring buyer-as-competitor multiplexity but does not affect the supplier's perception in the case of buyer-as-competitor multiplexity. Our results provide tactical insights as to when buyer participation helps or hurts.