The payment of reparations from the former Central Powers after World War I was among the most politically controversial parts of the Peace Treaties. An important part of these reparations, often overlooked in scholarship, was damage payments from the Central Powers to individual nationals of the Allied Powers. The Mixed Arbitral Tribunals (MATs) were tasked to decide such disputes over private damage claims advanced by Allied nationals or companies against former Central Powers governments. In doing so, the MATs were among the first international tribunals to ever allow individuals to claim private damages from foreign governments. This was stipulated in the peace treaties of 1919–23, such as the Treaty of Versailles with Germany. Starting in 1920, the MATs were established as international judicial bodies that combined several features that distinguished them from traditional ad hoc arbitral tribunals, notably their semi-permanent nature, their composition, their jurisdiction, and their rules of procedure. This system was innovative and spearheaded new pathbreaking techniques of international law. Taking the example of English and Polish claimants, this presentation will lay out the new relevance of international law for Allied nationals and their understanding of justice. Not in the least, this relevance was expressed in the sheer number of claims for reparation/compensation, with tens of thousands of claimants petitioning the MATs from all over the world.