Abstract Although advertisers present assurance or certification cues to burnish their “green” credentials, the impact of such “eco-seals” on persuasion is not well understood. We examine consumer characteristics (environmental concern and brand familiarity) and advertiser-controlled characteristics (the seal and advertising appeal) to understand conditions under which eco-seals are more or less persuasive, including effects on attitudes and intentions. Based on the Persuasion Knowledge Model (PKM), we hypothesize and present experimental results showing that consumers with high versus low environmental concern perceive eco-seals differently, depending on brand familiarity, eco-seal source, and ad appeal. Our findings have theoretical and practical implications for green marketing strategy and messaging.