Scholars have debated whether mediator impartiality or mediator interest plays a more vital role in bringing about a successful outcome. This research develops a comprehensive model that accounts for mediation occurrence and medication outcome in terms of an additive model of both mediator impartiality and mediator interest. The two channels through which mediators influence the changes of mediation and occurrence and outcome are hypothesized to be two dimensions of trust, mediator fairness and mediator capacity. This research argues 1) that mediator impartiality contributes to successful mediation outcomes by improving disputants’ trust in mediators’ fairness and 2) that mediator interest increases the likelihood of successful mediation outcomes by improving disputants’ trust in mediators’ capacity. Therefore, this research hypothesizes that the levels of mediators’ impartiality and mediators’ interest do not have individual effects on mediation occurrence and outcome. It argues, rather, that the additive level of the two variables determines the likelihood of mediation occurrence and successful outcome. The hypotheses of this research are tested using quantitative analysis of 294 interstate mediation cases carried out by states between 1945 and 1999 and qualitative analyses of the Philippines’ mediation of the Borneo dispute between Malaya and Indonesia in 1964 and Syria’s mediation of the border dispute between North Yemen and South Yemen in 1979. Both sets of analyses support the researcher’s claim that it is the additive level of impartiality and interest, rather than the individual level of both variables, that affects the chance of mediation success being achieved.