This thesis examines British attitudes towards the constitutional questions in Czechoslovakia and Yugoslavia. The dichotomy between centralism and federalism profoundly shaped the political affairs in the two successor states as well as the relations between their ethnic groups. In both cases, this would become a major destabilising problem that would play an important role in their eventual disintegration in 1939 and 1941 respectively. Given the impact it had on stability of some of the largest countries in an already volatile region, this issue could not escape the attention of British policymakers. Throughout the period, Britain was concerned with maintaining peace and order on the Continent. As such, appeasing disputes, even if entirely domestic, and finding solutions to resolve them became an important element of British foreign policy in the interwar period. When it came to Czechoslovakia and Yugoslavia, this by extension meant that the Foreign Office was forced to grapple with complex issue of federalisation in those two countries, particularly with regard to its potential to further consolidate the two successor states and therefore make them less susceptible to disintegration. This study analyses how federalism as a system of organisation was perceived in Britain between 1918 and 1939. By comparing and contrasting the British approach to federal demands in Czechoslovakia and Yugoslavia, it highlights a variety of factors which determined when federalism was seen as a viable state-building tool. In doings so, this thesis does not only re-examine the place that federalism occupied in British political discourse at a time but also situates it at the heart of British foreign policymaking, thereby providing a new perspective on the process of attitude formation in British governmental circles in the interwar period. It shows that whilst ultimately, federal schemes were not viewed as a universally ideal remedy for multi-national states, they were one that Whitehall repeatedly relied on when faced with instability and the prospect of conflict.