In this idiographic study, I examine language variation in the personal letters of individuals from two families with the goal of shedding light on the contribution they, and others like them, could have made to the formation of Early Modem Dutch urban vernaculars. The individuals selected for this study are of particular interest to historical sociolinguists due to their status as immigrants at a time (roughly 1580-1650) when urban centres in Holland experienced a doubling and tripling of populations. Using a single-genre corpus of autograph personal letters written by (im)migrants sharing similar demographic characteristics, I explore continuity and change in the linguistic repertoires of fourteen individuals who lived during a period of extreme social instability. I discuss three features (one phonological, one morphosyntactic and one syntactic) showing how some individuals' idiolects change as a result of their migration experiences while others do not, and I consider what this may mean for our understanding of language contact and change at this time.