Optimal commodity taxation with duty-free shopping

International travellers are frequently offered the opportunity to purchase a certain quantity of goods duty-free. Individuals differ in their opportunities to benefit from duty-free shopping, and we focus on the implications of these differences for optimal commodity taxation within a version of the optimal tax model of Mirrlees (Review of Economic Studies, 38, 175-208, 1971). We show how duty-free alters the constraints on the use of commodity taxes to reduce the distortionary costs of income taxation or to reflect externalities. Beyond characterising optimal taxes in the duty-free regime, we discuss conditions under which allowing duty-free would increase or reduce social welfare.