There is considerable evidence from the social loafing literature that groups can often undermine task motivation (relative to comparable individual performers). There is less but growing evidence that under the appropriate conditions, working in a group can have the opposite effect and actually produce a motivation gain. Little is known about how such motivation losses and gains are affected by the social relationships among group members. The present experiment examined the effect of being ostracized by one's work partner on the Köhler motivation gain (which occurs when less able team members work harder under conjunctive group task demands than when working individually). Such ostracism attenuated but did not eliminate the Köhler motivation gain. Ostracism only had such a moderating effect when participants worked in a group, not under comparable coactive work conditions. It is argued that social ostracism can undermine group members' concern for group success or for protecting their reputation in the group without affecting the social comparison processes that also contribute to the Köhler effect. Copyright © 2008 John Wiley & Sons, Ltd.