Many important phenomena within software engineering are difficult to define and measure. One example is software maintainability, which has been the subject of considerable research and is believed to be a critical determinant of total software costs. We propose using concept mapping, a well-grounded method used in social research, to operationalize the concept of software maintainability according to a given goal and perspective in a concrete setting. We apply this method to describe four systems that were developed as part of an industrial multiple-case study. The outcome is a conceptual map that displays an arrangement of maintainability constructs, their interrelations, and corresponding measures. Our experience is that concept mapping (1) provides a structured way of combining static code analysis and expert judgment; (2) helps in the tailoring of the choice of measures to a particular system context; and (3) supports the mapping between software measures and aspects of software maintainability. As such, it constitutes a useful addition to existing frameworks for evaluating quality, such as ISO/IEC 9126 and GQM, and tools for static measurement of software code. Overall, concept mapping provides a systematic, structured, and repeatable method for developing constructs and measures, not only of maintainability, but also of software engineering phenomena in general.