The province of Holland is the site of two major developments from the Late Medieval and Early Modern periods. Demographically, it rapidly evolved from an essentially agrarian area in the thirteenth century to one of the most urbanised regions in Europe by the end of the sixteenth century. During the same period the vernaculars of Holland underwent dramatic restructuring of their morphological systems resulting, we contend, from the influx of immigrants to nascent urban centers such as Leiden, Haarlem and Amsterdam and the outcome of centuries of ongoing dialect contact. In this paper, we focus on data from South Holland from 1300-1800 and the development of four linguistic features to argue that urban centers such as Leiden were the sites of ongoing koineisation processes. The linguistic developments are viewed as the consequence of the LI acquisition process of children confronted with heterogeneous input; as a result of the steady influx of immigrants to the urban centers of Holland that increased dramatically in the late sixteenth and early seventeenth centuries.