Affective instability is common during adolescence, but at high levels it is associated with a variety of internalizing and externalizing disorders, including eating disorders. Although most models focus on affective intensity as a mechanism for explaining eating disorders in adults, affective instability may be more developmentally relevant at predicting eating behaviors in youth. Using ecological momentary assessment, this manuscript explored the association between loss of control over eating (LOC), a key component of dysregulated eating in youth, and affective instability in youth with overweight/obesity in two separate studies, one with youth in middle childhood and early adolescence (Study 1: ages eight to 13) and one in youth in early through middle adolescence (Study 2: ages 12-17). Overall, there was no association between affective instability and LOC in Study 1, but in Study 2, age moderated the association between positive affective instability and LOC, such that greater between-person positive affective instability (i.e., relative to peers) was associated with lower average LOC for youth earlier in adolescence and higher average LOC for those later in adolescence. Negative affective instability was also associated with LOC in Study 2, such that on days when youth reported less within-person negative affective instability (i.e., relative to their own average), they also reported greater average LOC. Findings across the two studies indicate that the association between affective instability and LOC may not emerge until adolescence, and when it does, both positive and negative affective instability may be important to consider.