Systematisation of declarative process knowledge is required for development of process design and modeling methodologies. Declarative knowledge is structure, state and performance variables and relations between these variables, i.e. the ’what is ... ’ knowledge around a processing system. Object-oriented classification of process knowledge both facilitates modification and updating of data and ensures reusability of knowledge, which is encapsulated as objects. Reuse is made feasible by inheritance of knowledge and by associating objects with semantic relationships. This paper introduces the reader the basic object-oriented concepts. The principal notice in this paper has been focuses on abstracting object classes in the field of process engineering in general. Secondly, the inquiry has been concerned especially with chemical engineering. The abstracting of knowledge and the naming of objects have been made in terms of application domain. The object taxonomy has been decomposed in ’top down’ fashion and it has been implemented in an object-oriented programming environment.