Sexuality is a key component of health and functioning that changes with age. Although most sexual activity takes place with a partner, the majority of research on sexuality has focused on individuals. In this paper, we focused on the sexual dyad. We proposed and tested a conceptual model of the predictors of partnered sexual activity in older adulthood. This model began with the personality of each of the partners, which affects individuals' views of sex and characteristics of the partnership, which in turn affected sexual expression in the couple. We measured a key feature of personality, Positivity, which reflected the individual's tendency to present his or herself positively in social situations. This trait, we posited, increased frequency of sex through increased desire for sex, and the subjective importance of sex to each member of the couple. In this model, Positivity also impacted characteristics of the relationship that promoted dyadic sexual behavior. These processes differed for men and women in the model. We tested this model with data from the National Social Life, Health and Aging Project, which conducted personal interviews with both partners in 940 American dyads (average male age 72; average female age 69). We found that couples in which the husbands' (but not wives') were high in Positivity show higher levels of sexual activity, and that this association was partially mediated by dimensions of relationship quality, but more so by individual factors such as thinking about sex and believing sex is important.