The tone-changing rules in Central (Standard) Thai are traditionally analyzed as having five contrasting tones--mid, low, high, falling, and rising. (Abramson's graph of the fundamental frequency variations for single vowel Thai syllables illustrates these five tones.) Theoretically, each tone may be part of any Thai syllable. Any Thai syllable may be potentially five different morphemes, with the following restrictions--single vowel syllables ending in a stop have only high or low tones and geminate vowel syllables ending in a stop have only falling or low tones. Data is presented (in phonemic transcription) which suggests that there are several optional or stylistic tone-changing rules in Thai which are conditioned by the style and tempo of speaking. That is, these tones distinguish a careful isolative style from a more rapid combinative style of speaking. The author discusses Wang's matrix of seven features and 13 tones which may be used to describe any tone language. He feels that although Wang's proposal works "quite well in this area of Thai phonology and the correlation between the phonologically optimum representation and the acoustically most plausible is interesting," further investigation is needed. This paper will appear in UCLA's -Working Papers in Phonetics," Fall, 1968 issue. (AMM)