This study tests the output constraints hypothesis that languages with a crowded phonemic vowel space would allow less vowel-to-vowel coarticulation than languages with a sparser vowel space to avoid perceptual confusion. Mandarin has fewer vowel phonemes than Cantonese, but their allophonic vowel spaces are similarly crowded. The hypothesis predicts that Mandarin would allow more coarticulation than Cantonese. Eight native speakers of Cantonese and of Beijing Mandarin were recorded saying the target sequences /pV[subscript 1][superscript 1]pV[subscript 2]pV[subscript 3]/ (V = /i a u/) in carrier phrases. F1 and F2 frequencies were measured at vowel edge and midpoint, and were normalized for analyses. The results show that Cantonese and Mandarin do not differ in degree of vowel-to-vowel coarticulation in either F1 or F2. In addition, unstressed vowels exhibit more coarticulation than stressed vowels. Carryover coarticulation exceeds anticipatory coarticulation in both F1 and F2. Unstressed vowels in the carryover position are the most susceptible to coarticulation. The results show that vowel inventory does not predict vowel-to-vowel coarticulation. Fundamental assumptions of the output constraints hypothesis are evaluated to explain its failure in predicting language-specific patterns of vowel-to-vowel coarticulation. The importance of syntagmatic relationships in coarticulation is also discussed. (Contains 1 table and 6 figures.)