This paper applies a two-country framework that allows for currency substitution in an environment in which policymakers optimally vary interest rates in light of utility-based objectives, one country pegs the value of its currency to the other nation’s currency, and government revenue is generated via explicit taxes and seigniorage. The analysis illustrates the roles that currency substitution, currency preferences, and efficiency of tax systems play in contributing to the likelihood of a “run” on one nation’s currency. We explore how these factors interact to influence the probability of a currency crisis in the country that fixes its exchange rate.