This study examined trajectories of positive and negative affect surrounding penile-vaginal sexual intercourse (sex) in adolescents and explored the influence of companionship on these trajectories. Using a handheld computer, sexually active adolescents (N = 67) reported momentary affect and sex events in response to several random signals each day for 1 week (1,777 reports, 266 sex events). Cubic spline regression analyses showed that positive affect began to increase before sex, peaked at the time sex was reported, and then returned to baseline. Negative affect did not differ from baseline before sex, but decreased following sex. Improvement in affect before and after sex varied according to companionship. Understanding the complex associations of affect, companionship, and sex has implications for interpreting and intervening on adolescent sexual behavior in social context.