Speech enabled interfaces and spoken dialog systems are mostly based on statistical speech and language processing modules. Their behavior is therefore not deterministic and hardly predictable. This makes the simulation and the optimization of such systems performances difficult, as well as the reuse of previous work to build new systems. In the aim of a partially automated optimization of such systems, this paper presents a formalism attempt for the description of man-machine spoken communication in the framework of spoken dialog systems. This formalization is partly based on a probabilistic description of the information processing occurring in each module composing a spoken dialog system but also on a stochastic user modeling. Eventually, some possible applications of this theoretic framework are proposed.