A basic assumption of the research reported upon here is that measurable vowel duration is at least partly controlled by an independent phonetic feature of vowel length. We have studied manifestations of this feature by measuring articulatory segment durations of a number of Dutch short vowels, long vowels and diphthongs, in nonsense words of the form /pVpVpVp/ of which the second syllable was stressed. /V/ was the same vowel in each syllable. The /p/ durations and the total word durations were also measured. The durations were defined by the moments of lip opening and closure measured with a lip contact. The results can be explained by assuming that on a programming level of speech production there are only two possible specification values of the feature of vowel length for Dutch vowels, one for the short vowels /α, œ, I, ε, u, y, i/ and one for the long vowels /a:, o:, ø:, e:/, plus the diphthongs /αu, Λy, εi/. The programming level of speech production is taken to correspond to the linguistic level of phonetic representation. Durational variations within each of the two phonetic categories of vowel length can be explained by peripheral properties of the speech production system. Variations due to stress and position are accounted for by assuming that prosodic patterns may affect the amount of deviation of the actual duration from the specified ideal duration.