This paper studies the role of main stress in Brazilian Portuguese sandhi rules. It is shown that, when the second vowel of a VV sequence carries the phrasal stress, Degemination and Elision cannot apply to resolve the hiatus, whereas under the same circumstances, Diphthongization may occur. Within Optimality Theory, this restriction is explained by a Local Conjunction that controls vowel deletion as well as the alignment between the main foot and the head of the lexical word, which is irrelevant for the evaluation of outputs containing a diphthong, and therefore allows, in the latter case, the unmarked structure to emerge.