Investigation has been undertaken into the use of software specification methods for instrument systems specification. In the first part of this paper, some of the well-known structured software specification methods are briefly evaluated against a set of criteria we established, in the context of measuring instrument systems. We then conduct a case study in the widely used CORE method. It has been established that CORE can provide systematic and effective support for functional requirement analysis of complex instruments and instrument systems. Its decomposition-based analysis helps to create complete and consistent functional requirement specification.