The present study evaluates a mass media public information campaign regarding the greenhouse effect, designed by the Dutch Ministry of the Environment. During more than two months, the mass media (national television, national newspapers, billboards) were employed intensively to make the public aware of the nature and causes of the greenhouse effect, its consequences and possible ways of dealing with this environmental problem. As an evaluation of the campaign was deemed important, the research reported here was executed. In a Pre-Post Design survey (N=704), assessments were made of changes in knowledge, problem awareness, willingness to show ecologically sound behaviours and perceived necessity of additional superimposed policy measures. The results show that it is hard to change current cognitions and behaviour but, more importantly, the data suggest that knowledge and problem awareness may be less instrumental in promoting behavioural change than was assumed before the campaign. These results are reflected upon from a social dilemma perspective. This perspective stresses that it is unlikely that individuals are willing to forfeit some of their personal comfort in favour of long-term collective interests if not only the desired collective goal (the prevention of ecological disaster) but also the contributions made by other parties are surrounded by much uncertainty. The present campaign primarily focused on reducing citizens' uncertainty about the collective goal and ways of achieving it, while it left citizens in complete uncertainty about the contributions made by other parties. Additional analyses support the significance of citizens' expectations about contributions of other parties. These expectations were positively related to respondents' own contributions and negatively related to the perceived necessity of additional superimposed policy measures. The strategic implications for future campaigns are discussed.

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