This study examined the role of interpersonal communication in the context of a mass media anti-smoking campaign. Specifically, it explored whether conversations about campaign ads and/or about quitting mediated campaign exposure effects on 2 quitting behaviors (sought help to quit and tried to quit smoking completely), as well as the relation between ad-related and quitting-related conversations. Data were collected before the campaign and monthly for 16 months during the campaign through cross-sectional telephone surveys among a sample of 3,277 adult Philadelphia smokers. Follow-up interviews were conducted among 877 participants 3 months after their first survey. Cross-sectional and longitudinal mediation models with bootstrap procedures assessed the indirect effects of campaign exposure on outcomes through conversations, and the indirect effects of conversations about ads on outcomes through conversations about quitting. In addition, lagged regression analyses tested the causal direction of associations between the variables of interest. The results partially support hypotheses that conversations about quitting mediate campaign effects on quitting-related behaviors and, in line with previous research, that conversations about the ads have indirect effects on quitting-related behaviors by triggering conversations about quitting. These findings demonstrate the importance of considering interpersonal communication as a route of campaign exposure effects when evaluating and designing future public health campaigns.