This paper examines the political determinants of income composition inequality in 32 advanced and emerging economies between 2006 and 2018. Income composition inequality is defined as the extent to which the income composition in capital and labor income is unevenly distributed across the income distribution. High levels of income composition inequality are associated with class-fragmented societies, whereas low levels are typical of multiple-sources-of-income societies. We find that a higher seat share of left parties in the governing coalition and higher globalization, as measured by trade, capital openness, and FDI inflows, are linked to lower income composition inequality. Higher economic development and a higher capital income share are, instead, related to higher inequality in income composition. We discuss the mechanisms behind these relationships and check the robustness of our findings. To our knowledge, this is one of the first studies looking into the causes of the dynamics of this dimension of economic inequality. (Stone Center on Socio-Economic Inequality Working Paper)