We explore the implications of monetary unification for real interest rates and (relative) public debt levels. The adoption of a common monetary policy renders the risk-return characteristics of the participating countries more similar, so that the substitutability of their public debt increases after unification. This implies that the average expected real return on the debt increases. Also, the share of the unionwide debt issued by relatively myopic governments or of countries that initially have a relatively dependent central bank increases after unification. This may put the political sustainability of the union under pressure. A transfer scheme that penalizes debt increases beyond the union average is able to undo the interest rate effect of unification, but magnifies the spread in relative debt levels.