This paper describes and analyzes two different sub-systems of knowledge structures - namely “domain-oriented” and “solving-oriented” knowledge structures - in the results of a case study based on interviews and experimental sessions. This case study consisted in the analysis of the activities of an expert programmer in his debugging of a complex computer system. The paper describes the approach adopted to study the conceptual (domain-oriented) structures in relation to the cognitive (solving-oriented) patterns in the context of problem solving activities. The results appear to be consistent with the role of some solving-oriented cognitive patterns in debugging activity and the contextual effects of the problem on the conceptual structures.