The number of websites containing persuasive serious games and advergames has increased over the past several years, but their immediate and delayed effects on behaviour are still not well understood. The present field experiment (n=388, varied socio-professional groups) demonstrates that interactivity linked to this type of website provokes positive effects on immediate behaviour (purchases of energy-saving light bulbs –ESLBs– ) in a “real setting”. It further affected the behaviour (installation of ESLBs at home), the memorization of the website's arguments, gains in knowledge, attitude, and other judgments regarding ESLBs, when measured two weeks later. The digital signature of a commitment to perform an expected behaviour via a Web page also provokes positive behavioural effects. This can accumulate through the effects of interactivity. We close with a discussion of the possible psychological processes involved, theoretical and practical implications and limitations as well as new perspectives for advertising and advergames research.