Stamp duty, the oldest Portuguese tax, must be carefully considered in the context of transactions involving both internal and cross-border loans and/or guarantees. Created on 24 December 1660, stamp duty has been subject to several amendments. In 2000, in an attempt to bring this archaic tax up to date, a new stamp duty law was introduced. This article examines some transactions involving cross-border loans and guarantees and the impact that stamp duty has on each of them.