This paper presents the processes of knowledge acquisition and ontology construction for developing an expert system for monitoring and control of natural gas pipeline operations. Knowledge acquisition is the process of eliciting the cognitive mechanisms an expert uses in solving a particular application problem. And an ontology captures the shared conceptualizations that experts have about a particular area. Knowledge on the problem domain was acquired and analyzed using the Inferential Modeling Technique. The analyzed knowledge was subsequently organized into an application ontology and represented in the Knowledge Modeling System. By formally representing the acquired knowledge in an application ontology, systematic organization of the knowledge elements implicit in the problem of monitoring and control of natural gas pipeline operations is achieved. The application ontology constructed can provide the basis for development of an expert system that can function as a decision support system for pipeline operators.