In January 1999, the European monetary union (EMU) was formally launched with 11 member countries. However, before May 1998 there was considerable uncertainty about who would join EMU, and whether the project would start on time. When a monetary union is formed, exchange rates between the member countries are irrevocably fixed, and yield spreads stemming from exchange-rate risk are eliminated. As a direct consequence, EMU affected the prices of long-term bonds well before 1999, but quantifying this effect can be difficult when there is uncertainty about the monetary union. We address these issues and develop a bond-pricing model which explicitly takes into account that a country may join a monetary union at a future, unspecified date. The empirical results show that a narrow EMU, consisting of Germany, France and the Benelux countries, has been priced with almost 100% probability throughout the period 1995–1998, whereas, on average, the implied probability of joining EMU has been somewhat lower for the other EU countries. However, in the period leading up to May 1998, the estimated probabilities have increased considerably for the countries that joined EMU in January 1999.