The game theory literature interprets the EMS as a disinflationary monetary regime merging a rule with an escape clause. This interpretation does not explain why the rule is implemented and it does not consider the imperfection and the asymmetry between the information sets of the players. In this paper these features are explicitly considered. The EMS is modeled as a flexible "institutional" framework within which the game between the private sector and a committed low-inflationary policy maker takes place. The model provides a simple framework for estimating the credibility of a country's participation in the EMS, without relying on particular stochastic assumptions on the process driving the fundamentals or the expectations of realignment. It shows also that the member countries can in the short run enjoy some monetary independence, without losing the credibility of their commitment. In the second part of the paper, an estimate of the credibility and of the degree of monetary independence exploited by the EMS countries is derived.