Inadequate definition of key terms and their relationships generates significant communication and analytical problems in environmental planning. In this work, we evaluate an ontological framework for environmental planning designed to combat these problems. After outlining the framework and issues addressed, we describe its evaluation by a group of experts representing a range of expertise and institutions. Experts rated their level of agreement with 12 propositions concerning the definitions and models underpinning the framework. These propositions, in turn, were used to assess three assumptions regarding the expected effectiveness of the framework and its contribution to addressing the abovementioned planning problems. In addition to point-based best estimates of their agreement with propositions, expert ratings were also captured on a continuous interval-valued scale. The use of intervals addresses the challenge of measuring and modelling uncertainty associated with complex assessments such as those provided by experts. Combined with written anonymous expert comments, these data provide multiple perspectives on the level of support for the approach. We conclude that the framework can complement existing planning approaches and strengthen key definitions and related models, thus helping avoid communication and analytical problems in environmental planning. Finally, experts highlighted areas that require further development, and we provide recommendations for improving the framework.