This paper addresses an interesting theoretical intuition put forward in De Grauwe (1989), according to which market efficiency should find stronger support in a pegged exchange rate regime rather than in a purely floating context. To this purpose, we extend to a selected group of long term EMS currencies the empirical investigation carried out in Tronzano (2002). As revealed by cointegration tests, a formal exchange rate commitment exerts a significant positive effect on market efficiency, although the above result cannot be generalized to all EMS currencies. Whenever the credibility of a target zone is relatively low, the link between macroeconomic fundamentals and the exchange rate becomes highly uncertain, giving rise to a time-varying risk premium component invalidating forward rate unbiasedness. This evidence has relevant policy implications on some reform proposals of the international monetary system which aim at preventing large exchange rate misalignments among the leading world currencies.