This study examined the interaction of ambient frequency and feature complexity in the diphthong errors produced by Cantonese-speaking children with phonological disorders. A total of 611 diphthongs produced by 13 Cantonese-speaking children with speech disorders were subjected to perceptual analysis. The percentage accuracy of production and error patterns was examined. Perceptual analysis showed that /(see text)i/ and /ui/ were most frequently in error, whereas /ei/, /ou/, and /(see text)u/ were least frequently in error. Diphthong errors (usually diphthong reduction) arise as a function of both ambient frequency and feature complexity. The combination of ambient frequency and feature complexity yields a complexity metric reflecting accuracy of production. Treatment guidelines include consideration of three basic factors: ambient frequency, feature complexity, and error patterns.