In this technical report we define an operational net semantics for the stochastic process algebra MPA based on generalized stochastic Petri nets. As a consequence of the definition of such a semantics, we present an interesting modeling technique for concurrent systems which is based on MPA. 1. Dipartimento di Matematica, Università di Bologna, Piazza di Porta S. Donato 5, 40127 Bologna, Italy.