Material data are of basic importance for the design, operation, and maintenance of process plants. To support computer-based modeling, calculation, and management of material data, a conceptual data model is required. Although the issue of material data modeling has been addressed in a number of efforts, a model, which is concise, self-contained, and well structured, is still not available. In this work, a systematic categorization of material data has been performed. This leads to a conceptual material data model consisting of three partial models, namely 'substance', 'phase system', and 'phase system mathematical model', each representing a separate perspective of material data in accordance with the physical-basis as well as practical uses. This conceptual data model can be used for the development of detailed data models that are suitable for specific applications by making extensions and refinements to it. Such applications of the conceptual model. are explained by the experiences gained in two software development projects.