In this paper we propose an alternative explanation for the nature, sources and consequences of inflation rate differentials in a monetary union, such as EMU. To achieve this, we build on the new neoclassical synthesis (NNS) framework, recently advanced by Goodfriend (2002) and Goodfriend and King (2000). Based on the NNS setup, we discuss the inflationary consequences of the catching-up process in a heterogeneous monetary union. In particular, we explore the interaction between catching-up and inflation differentials and offer an interpretation of the nature of this interaction. Our discussion is in stark contrast to the conventional Balassa-Samuelson (BS) interpretation. In particular, we demonstrate that divergent inflation rates between Member States do not necessarily have to be an equilibrium phenomenon, even if the original shock comes from the supply-side of the economy. Second, we show how a centralized monetary policy may produce such divergence of individual country's inflation rates when countries differ in size and in trend productivity growth. Against this background, we additionally show how the catching may potentially lead to unsustainable credit booms in a catching-up member country. Finally, we indicate some important deficiencies of the BS model as a guide to short- and medium-run policy making analysis.