Teaching mathematics in informal settings is a relatively new phenomenon, but it has gained more attention due to the recent changes in the society. The aim of the present quantitative study was to compare the learning outcomes of Latvian and Swedish 12-year-olds when they visited a science centre mathematics-art exhibition originally designed in Estonia. The results showed that in general, prior knowledge of the exhibition contents was the strongest predictor of post-test results in both countries but that mathematical thinking skills and self-concept had a small added value in explaining the post-test results. The results of the study give some of the first pieces of evidence of the effectiveness of out-of-school mathematics teaching in a science exhibition context, providing a good basis for further studies.