Increasing knowledge is often set as the objective of risk communication. But is it worthwhile focusing risk communication strategies solely on enhancing specific knowledge? The main research questions tackled in this paper were: i) if prior knowledge related to specific radiation risks is influential for the perception of these risks and the acceptance of communicated messages and ii) if gender, attitudes, risk perception of other radiation risks, confidence in authorities and living in the vicinity of risk installations may also play an important role in this matter. The intent of this study was to empirically test these predictors in two independent case studies in different countries. The first case study was an information campaign for iodine pre-distribution in Belgium (N=1035). The second was the information campaign on long term radioactive waste disposal in Slovenia (N=1200). In both cases, recurrent and intensive communication campaigns were carried out by the authorities aiming, among other, at increasing specific knowledge. Results of regression analysis show that higher prior knowledge leads to more willingness to accept communicated messages, but it does not affect people's perception of the specific risk communicated.