The Chemical Process Industry is been forced by the society to improve its standards in safety, environmental effects and product quality. One approach used to address all these aspects in traditional process industries is the HAZOP study (HAzard and OPerability study). But, the application of this technique to a detailed chemical plant design is a complex and time consuming task. These two aspects and the availability of proved technology to develop computational systems able to manage qualitative knowledge, have motivated the construction of SERO, a knowledge-based system for HAZOP studies. The objective of this project is to generate a system aimed at enhancing the knowledge and creative power of the expert team involved in a HAZOP study. One of the main aspects of SERO, that is presented in this paper, is its knowledge representation structure, which has been developed to encode the different kinds of knowledge used in a HAZOP study. The knowledge representation structure of SERO has been designed and constructed using the object oriented paradigm.