Phonological descriptions of the Standard Dutch vowel system distinguish between tense and lax vowels. Both categories may differ in intensity, vowel quality and duration. This project focuses on the perceptual role of duration in vowel categorisation for tense vowels. We study how differences in vowel duration are perceived by listeners with different regional backgrounds. The durational measurements are made in spontaneous speech samples of native speakers in well-defined geographical regions of Flanders (Belgium) and the Netherlands.