In this paper I discuss the acquisition of three external vowel sandhi rules (elision, diphthongization, and degemination) in Brazilian Portuguese, based on production data by two children from 1;4 to 3;6 years old. I argue that the acquisition of sandhi rules is dependent on the acquisition of syllable structure. Children are shown to incorrectly apply elision over diphthongization when they start applying sandhi rules, because they do not master the syllable structures that allow them to produce diphthongs. I also show that the properties of the rules are not mastered simultaneously and that the properties related to higher prosodic domains are the first ones to be acquired.