The aim of this study is to investigate the relationship between high school students' understanding of carbon compounds, including their transformations, and their ability to trace matter in the carbon cycle. The rationale of this study is based on the hypothesis that the students' lack of knowledge about carbon compounds and their transformations limits the students' ability to trace carbon in the carbon cycle. A total of 16 students aged 14-16 years (grade 9) from two different secondary schools were interviewed about carbon compounds, and they were asked to trace matter in the carbon cycle in response to a written diagnostic task. We argue that students' lack of knowledge about carbon compounds and their transformations is responsible for their construction of the carbon cycle as either a gas-gas or solid-solid cycle. Educational implications are discussed.