The decline in infant mortality played a crucial role in the health transition in the Western World. This decline among the vulnerable new-borns was however not an evenly dispersed process. Inequalities as a result of regional differences, cultural influences or socioeconomic status shaped the paths towards low mortality rates. The role of socioeconomic status in levels of infant mortality and its decline remain highly debated. In this article, we study the development of socioeconomic disparities in infant mortality in the Dutch town of Maastricht in the period 1864-1955. This study uses unique individual-level cause of death data in order to see when changes in disease patterns took place for different socioeconomic groups. The aim is to identify socioeconomic inequalities by mapping changing epidemiological patterns over time, according to age within the first year of life. By deploying a multinomial logistic regression analysis we can trace the different timing of changes in the epidemiological regime. The results reveal that for most infants the change in mortality patterns from water- and foodborne infectious diseases towards a predominance of airborne infectious diseases occurred simultaneously with the massive decline in infant mortality from the start of the First World War onwards. Infants from the upper classes, however, appeared to have gained an earlier advantage, followed by infants from unskilled workers. Finally, from qualitative data it becomes clear that the awareness of the problematic nature of infant mortality, knowledge on infant care, hygienic practices, breastfeeding practices and the economic situation of World War I were factors aiding to the uneven decline in infant mortality.