An analysis of vowel distribution patterns in Rawas, an Austronesian language spoken in Sumatra, is presented. The language has six vowels, each with long and short variants in complementary distribution, and five diphthongs that occur generally in open syllables. It is proposed, based on this analysis, that each Rawas syllable is precisely two moras long, and that vowel distribution derives from this condition. The condition can be fulfilled in three ways: by a lexical closed syllable, by a lexical diphthong, and by a syllable whose melodic content is consonant-vowel which surfaces with a single long vowel. (MSE)