This paper focuses on the representation of the knowledge about users, products, and tasks that a product support system should contain. Knowledge is acquired following a three stage process of transforming raw data into information and knowledge by the utilisation of ontological principles. New notions are introduced (domain, connector, knowledge-specifier, and arc) for formalising the acquired knowledge and applying it in the context of product support. A new measure (connector-dependence) is also proposed to ensure the completeness and validity of the developed structures. In addition, a knowledge base has been developed based on the knowledge model described. This new approach for representing product support knowledge formalises the context within which a product support system operates and improves the product support responsiveness by prescribing the knowledge sets used and how these should be represented. The knowledge model provides an ontology that can be shared and reused by product support systems. It introduces the structural components needed, the links between them, and establishes their usability. The developed ontology advances the interoperability between product support and other knowledge intensive fields, by representing the elements of knowledge in a machine processable way. Furthermore, it facilitates the provision of adaptive and personalised support. This work is a step towards formalising product support as a knowledge intensive process. Future work includes the development of a product support framework, where the developed knowledge base will be one of the major components.