This article analyses Thailand's place in Japan's Greater East Asia Co-Prosperity Sphere and how Japan financed its goal of integrating the kingdom into the sphere. Financial arrangements to incorporate Thailand in a yen bloc go well beyond finance to reveal Japanese attitudes and policy towards the Co-Prosperity Sphere. In Thailand, Japan's use of ‘special yen’ created near open-ended Japanese purchasing power. Japan could obtain whatever resources it could ship home but provide Thailand almost no goods in exchange. Although in response to Japanese demands the Thai government printed large quantities of money, prices rose not too much faster than monetary expansion. Thailand, unlike most of wartime Southeast Asia, avoided hyperinflation. It is argued that principal explanations for this economically unexpected stability were Thailand's particular economic structure and the behaviour of Thai peasants.