This research examines the effect of mediators’ characteristics on mediation outcomes. In the current study of international mediation, one group of scholars argues that biased mediators with a considerable interest at stake in the dispute are usually effective. Others stress that mediator neutrality is a precondition for mediation to be successful. To test these claims, this study evaluates the Philippines’ qualifications as a mediator for the Borneo confrontation between Indonesia and Malaya in the 1960s. Although the Philippines’ strong ties to both disputants qualified it as an impartial mediator, its ongoing dispute with Malaya over Sabah transformed the Philippines into a biased/interested mediator in the Borneo confrontation. This research illustrates how effective the Philippines was in mediating the Borneo confrontation. It also sheds light on the possible futility of South Korea’s involvement as a third party in the People’s Republic of China-Japan territorial dispute.