Nikolaus Ludwig von Zinzendorf (1700–1760) is often remembered for his ecumenical theology. Yet his relationships with other Christians of his time were marked by conflict, and every significant ecumenical connection he made was eventually broken off. This article outlines Zinzendorf's interactions with other Christians in the two centres of Moravianism where his leadership was strongest, Germany and England, and analyses the consistent disintegration of those relations. It concedes that these conflicts were fuelled in part by suspicion of Zinzendorf's radical ideals, fear of his movement's independence, the ecclesial politics of his time, the public's appetite for gossip about the Moravians, and the faults of his conversation partners — all causes that are often invoked to explain eighteenth-century antipathy toward Zinzendorf. The far more consistent and compelling factor in these conflicts, however, was Zinzendorf's temperament, which included both a noble sense of being above reproach and a distinct irritability. This article argues that Zinzendorf's contentious personality was the decisive impediment to the realisation of his ecumenical goals. It also suggests that his tendency to be a controversialist helps make sense of the contradiction between his ecumenical theology and the failure of his ecumenical program under his leadership.