In this paper, three experiments are reported that were run in order to assess the quality of Dutch synthetic speech using accented and unaccented diphones, i.e., diphones extracted from accented and unaccented syllables, respectively. In a paired-comparison design, subjects were asked to evaluate the naturalness and fluency of different versions of an utterance. The results of the first two experiments, in which isolated polysyllabic words were used, indicate that the use of accented or unaccented diphones has a perceptual effect on phonologically long vowels only. In a third experiment, the use of the different diphone types in short sentences with a fixed temporal structure was evaluated. Results suggest that using unaccented diphones in unaccented or secondarily accented syllables does not result systematically in more natural-sounding speech.