Logical reasoning as part of critical thinking is becoming more and more important to prepare students for their future life in society, work, and study. This article presents the results of a quasi-experimental study with a pre-test-post-test control group design focusing on the effective use of formalisations to support logical reasoning. The participants were 163 pre-university secondary school students (11th and 12th graders). For the experimental group we designed a course in logical reasoning intending to develop students’ use of formalisations in formal reasoning and everyday reasoning tasks. We show that students learned to use the intended formalisations. Overall, students from the experimental group had significantly better results on the post-test compared to the control group. Positive correlations were found between item scores and the use of Venn and Euler diagrams and scheme-based strategies. Our results support our hypothesis that the use of formalisations improves logical reasoning skills.