Acoustic analyses of vowel error patterns in five children with DAS validated the perceptual judgements described in a study by Pollock and Hall (1991). Results of the present study showed that perceptual judgements of diphthong reduction corresponded highly with second formant (F2) movement ratios of 0·20 or less. Perception of correct diphthong production was perceived with F2 ratios of 0·23 or greater for θ, 0·24 or greater for θ, and 0·36 or greater for θ. The perception of laxing and tensing errors for high vowels often corresponded to the location of F1-F2 coordinates. The perception of laxing of/e/, however, was more often associated with F2 ratios of less than 0·20, indicating a lack of change in vocal tract configuration or a loss of diphthongization. Derhotacization errors were perceived when there were abnormally high F3 frequencies and F2 and F3 coordinates that were far removed from the mean for normal children. These results demonstrated the fact that, for these data, perceptual judgements were a valid and reliable means of assessing vowel and diphthong productions.