Rooted in commitment theory, this study examines dedication, perceived investments, and felt constraint, three unique aspects of commitment, in midlife using data gathered from a sample of 224 adults surveyed at ages 43 and 50 years. Positive and negative dimensions of couple relations (partner supportiveness and the extent to which one's partner is demanding and critical, referred to as strain) assessed longitudinally and midlife-specific circumstances, including being in a first marriage or remarriage and parental and empty nest status, were examined as predictors of commitment at age 50. Latent change score modeling revealed partner support and strain as robust predictors of commitment. Higher levels of partner support at age 43 and a more gradual intraindividual decrease in support between ages 43 and 50 predicted higher dedication and perceived investments and less felt constraint at age 50. Higher age 43 levels of partner strain predicted more perceived investments and felt constraint at age 50, while a more rapid increase in strain predicted less dedication and more felt constraint. Being in a first or remarriage, being a parent, or having an empty nest did not predict commitment. These results highlight the importance of initial levels and changes in couple relations for shaping midlife commitment.