This paper describes the design of an instrument to assess secondary school students' proficiency in structure-property reasoning (SPR). Design criteria for the instrument required that it should be based on a comprehensive model for structure-property reasoning, assess both reproductive and productive use of structure-property reasoning, be cost-effective, and be easy for teachers to adapt to their situation. An unframed and framed sorting task and an unframed and framed mapping task were included in the instrument. It was used to determine the proficiency in structure-property reasoning of two populations: 60 Dutch secondary school students on the preuniversity track and 108 Dutch first-year university chemistry students. Results were analyzed using established statistical techniques, and they confirmed that the SPR-instrument clearly discriminates between preuniversity and first-year chemistry students. The paper concludes by outlining the possibilities offered by the instrument and suggestions for further research.