We examined whether seven relational constructs (satisfaction, commitment, closeness, conflict, ambivalence, maintenance, and love) showed significant within-person residual variance over time (variability), whether couple members were similar in their variability (covariation), and whether variability and covariation differed by relationship length. We used dyadic daily diary data from 157 couples together from 2 months to 44 years. Each relational construct significantly varied within person from day to day, and couple members significantly covaried. Longer term couples generally experienced less variability than newer couples. We also found that for satisfaction, closeness, ambivalence, and love, couple covariation decreased in longer relationships. We discuss how these findings can be used to help individuals work to maintain greater stability in their feelings about the relationship.