In this paper we deal with the problem of determining a set of decentralized controllers for P/T nets that are able to impose a given global specification on the net behaviour. More precisely, both the global specification and the decentralized specifications are given in terms of generalized mutual exclusion constraints (GMECs) thus the controllers take the form of monitor places. In this paper we provide some preliminary results that are a first step towards a more general and systematic approach to the problem. The lines of our future research in this topic are described in details in the last section, devoted to conclusions and future works.