This study investigates the social-cognitive predictors of health behaviors, specifically preventive nutrition, among 580 adults at two time points. The authors hypothesized that optimistic self-beliefs operate in two phases: preintention (action self-efficacy) and postintention (coping self-efficacy). Risk perceptions, outcome expectancies, and action self-efficacy were found to predict behavioral intentions at Wave 1, while behavioral intentions and coping self-efficacy mediated the relationship between these predictors and low-fat and high-fiber dietary intake at Wave 2. Covariance structure analysis showed good model fit for the total sample and six subsamples created by median splits of gender, age, and body weight. Parameter estimates differed between samples, with perceived self-efficacy increasing importance with age and weight. The study supports the role of phase-specific self-efficacy and highlights the need for tailored interventions that consider individual differences in self-efficacy and other social-cognitive factors.This study investigates the social-cognitive predictors of health behaviors, specifically preventive nutrition, among 580 adults at two time points. The authors hypothesized that optimistic self-beliefs operate in two phases: preintention (action self-efficacy) and postintention (coping self-efficacy). Risk perceptions, outcome expectancies, and action self-efficacy were found to predict behavioral intentions at Wave 1, while behavioral intentions and coping self-efficacy mediated the relationship between these predictors and low-fat and high-fiber dietary intake at Wave 2. Covariance structure analysis showed good model fit for the total sample and six subsamples created by median splits of gender, age, and body weight. Parameter estimates differed between samples, with perceived self-efficacy increasing importance with age and weight. The study supports the role of phase-specific self-efficacy and highlights the need for tailored interventions that consider individual differences in self-efficacy and other social-cognitive factors.