This article provides novel estimates of long-term income inequality in Bulgaria and Czech Lands/Czechoslovakia in the twentieth century. Relying on newly-constructed datasets and the social tables approach, we measure inequality between salient social strata. We find that Czechoslovakia was significantly more unequal than Bulgaria before 1945. Inequality converged to similarly low levels under socialism. Decomposition analysis by social classes reveals that different levels of inequality in the first half of the century were principally driven by higher within social-class inequality in Czechoslovakia, owing to a more stratified industrial society; whereas a low dispersion within the dominant agricultural sector held down the within social-class component in Bulgaria. A dramatic fall in total inequality after 1945 was a result of the social revolution that encompassed the virtual disappearance of between social-class inequality and a marked reduction in within social-class inequality. Our findings point to the critical role of institutional and political factors in driving inequality in Eastern Europe throughout the twentieth century.