Abstract I use a dynamic general equilibrium two-country optimizing model to analyze the implications of international capital mobility for the short-run effects of monetary policy in an open economy. The model implies that the substitutability of goods produced in different countries plays a central role for the impact of changes in the degree of international capital mobility on the effects of monetary policy. Paralleling the results of the traditional Mundell-Fleming model, a higher degree of international capital mobility magnifies the short-run output effects of monetary policy only if the Marshall-Lerner condition, which is linked to the cross-country substitutability of goods, holds.