The present paper describes a study developed with the purpose of inspecting the intrinsic acoustic properties of vowels of stressed syllables in European Portuguese (with minor considerations about semi-vowels as well). The features we studied are fundamental frequency and duration. As in other languages, to some extent, these properties depend on vowel quality and context. The study addresses this issue. It is shown that not all correlations that have been discovered for other languages hold as well, but nevertheless these properties do differentiate vowels and diphthongs, and, interestingly, in some cases the lack of those correlations is offset by other correspondences.