The purpose of this paper is twofold. First, it introduces a novel inequality concept, named income composition inequality. Second, it constructs an indicator for its measurement. This paper argues that the study of income composition inequality across the income distribution allows for (i) novel political economy analysis of the evolution of economic systems and (ii) the technical assessment of the relationship between the functional and personal distribution of income. Following an empirical application on six European countries, this paper discusses possible avenues for future research on the matter, ranging from development issues to public finance. (Stone Center on Socio-Economic Inequality Working Paper)