The paper describes a possibility of LOTOS language application for modelling and verification of reactive systems in specification and design stages of development process. The main idea lies in an examination of behavioural equivalences between two specifications related to different abstraction layers, or between an ideal fault-free model and system (or its part) specification in current development stage. A framework for using LOTOS and observational equivalence for verification of artifacts in the above mentioned development stages is proposed in the paper. The concept is examined on example presented in the final part.