<p>Income composition inequality (ICI) refers to inequality in the distribution of different income sources across the personal income distribution. Although the concept has received increasing attention in the literature, the relationship between income composition inequality and overall personal income inequality remains largely unexplored. This study examines the role of ICI in the evolution of primary and secondary personal income inequality by introducing a new measure of ICI and analysing its impact on income inequality in Spain from 2007 to 2021. The proposed approach allows ICI to be measured across multiple income sources while distinguishing between primary and secondary income. Using econometric analysis, the study links changes in income inequality to variations in ICI across the income distribution, while also controlling for other socio-economic factors. The results reveal a strong relationship between ICI and personal income inequality, showing that increases in the ICI correlate with greater inequality in primary income distribution over the analysed period. However, the increase in primary income composition inequality in Spain has not translated into higher inequality in the distribution of secondary income, largely due to the strengthened redistributive role of the state during the analysed period. These findings highlight the need for more effective policies aimed at achieving a more equitable primary income distribution, particularly through wealth and financial income taxation.</p>