Prior research has established that parents who are in a relationship, yet unmarried at the time of their child's birth, are at an increased risk of relationship instability. However, the processes that may lead to the dissolution of these unmarried parents' couple relationships are less clear. Guided by the vulnerability-stress-adaptation model, the present study examined data from a sample of 1,575 mother and father dyads who participated in the Future of Families and Child Wellbeing Study over a 9-year period. A mixed effects Cox regression model was used to investigate how unmarried parents' reports of enduring vulnerability (depressive symptoms) over time influenced the onset of relationship dissolution between the time their focal child was 1- and 9-years old. Further, the potential mediating effect of mothers' and fathers' reports of stressful events (parenting stress) and adaptive processes (couple relationship interactions and coparenting behaviors) on the association between depressive symptoms and relationship dissolution by the 9-year follow-up were also examined. Results indicated that mothers' and fathers' reports of experiencing depressive symptoms over time were associated with relationship dissolution. Further, perceptions of couple interactions emerged as a significant mediator at the 3- (mothers) and 5- (mothers and fathers) year follow-up. Coparenting behaviors were a significant mediator for mothers and fathers at the 3- and 5-year follow-up. These results highlight how experiencing depressive symptoms over time, as well as perceptions of couple interactions and coparenting behaviors throughout the early years of parenting, are salient factors in the instability of unmarried parents' relationships. (PsycInfo Database Record (c) 2024 APA, all rights reserved).