This paper presents an approach to developing a unified conceptual model to describe and explain change in organisations, viewed as complex systems. The authors propose a model that brings together the traditional open systems model (based on principles of homeostasis, steady state, and cybernetics) and the dissipative systems model (based on thermodynamic non-equilibrium principles) to explain distinctively different phases of change. Gradual and incremental change can be explained by using the traditional open systems model, whereas dramatic and discontinuous change can be explained by the adoption of the dissipative systems model. These two phases of change occur naturally, depending on the nature and pattern of external and internal disturbances. Since the implementation of any information system involves some degree of organisational change, it would be valuable to the IS community to more clearly understand organisational change processes, thereby increasing the possibility of success.