This paper estimates a dynamic stochastic general equilibrium model of a currency union with nominal rigidities to explain the sources of inflation differentials between the Economic Monetary Union (EMU) and one of its member countries, Spain. The paper finds that productivity shocks account for 85% of the variability of the inflation differential. Demand shocks explain a large fraction of output growth volatility but not variability in inflation differentials. In addition, the estimated model finds evidence that inflation dynamics are different across countries in the nontradable sector only. Finally, the Balassa-Samuelson effect does not appear to be an important driver of the inflation differential during the EMU period. Copyright (c) 2009 The Ohio State University.