Abstract
Petri Box Calculus (PBC) is an algebraicmodel for the description of concurrent systems and sPBC (stochastic Petri Box Calculus) is a Markovian extension of that model. In this paper we add immediate multiactions to sPBC in order to increase the description power of this language. Thus, we both have timed multiactions that follow an exponential distribution, and multiactions that do not require any time and can be immediately executed. The denotational semantics of this model is based on a special class of GSPN (Generalized Stochastic Petri Nets), called gs-boxes.