This paper addresses the problem of integration of Petri nets representing manufacturinfg systems. A modular modelling methodology being adopted, we define a class of Petri nets that characterizes a large sort of real-word manufacturing systems. These Petri nets are shown to have the properties of liveness, consistency and reversibility. We also show that the integration of such Petri nets preserves these properties under fairly weak conditions. Moreover the integrated Petri net belongs to the same Petri net class. The results obtained in this paper are expected to shed light on the design, management and control of large scale manufacturing systems where complexity problems are bound to rise and can be dealt with from the point of view of modular modelling and integration.