The purpose of the present study was to assess primary students’ knowledge of ocean acidification and the carbon cycle before and after a teaching intervention and provide important information whether the introduction of ocean acidification in a carbon cycle context can help young learners understand the human impact on this complex environmental problem. Eighty-five students from two public primary schools in Greece participated in the intervention and answered a structured questionnaire and a self-assessment tool before and after it. The statistically significant improvement in students' mean scores on the knowledge test and the statistically significant increase in their perceived knowledge reveal that the intervention successfully improved their factual knowledge and perceived understanding of the carbon cycle and ocean acidification. However, some concepts and terms remained challenging for students.