The main purpose of this theses is the comparative study of the phonological systems and the sounds of Rayong sub-dialects :Amphur Muang, Amphur Ban Khy, Amphur Klaeng, Amphur PluakDaeng and Ban Chang to see their similarities and deferences. To be able to compare these Rayong sub-dialects, the researcher had to study the phonological systems of each Rayong sub-dialect. The phonological analyses were based on the linguistic data collected from 5 informants in the areas. Each of these 5 informants was selected by the researcher as the representative of each Rayong sub-dialect. The comparative study has shown that Rayong sub-dialects have both similarities and differences. Most of the Rayong sub-dialects have 20 consonant phonemes, whereas only 2 Rayong sub-dialects have 21. In general these consonant phonemes are phonetically similar except for /r/ where there are differences. There are 10 clusters in the Rayong sub-dialects except for Amphur Muang sub-dialect which has only 4. Each Rayong sub-dialect has 9 final consonant phonemes, 18 single vowel phonemes and 3 diphthongs. There is no phonetic difference in each of these types of sounds when all the sub-dialects are compared. As for tones, only Amphur Ban Khay sub-dialect has 6 tones while the others have 5. The phonetic shape of each tone is largely similar. The difference lies in the level of pitches for each tone shape. Besides, Amphur Ban Khay sub-dialect shows a difference in the relationship between the tones and the initial consonants from the other dialects. The content of the thesis is divided into 8 chapters. The first chapter is the introduction which states the problem, the purpose, the scope and the introduction which state the problem, the purpose the scope and the process of the research. The second to the sixth chapter deal with the phonological systems of Amphur Muang, Amphur Ban Khay, Amphur Kleng, Amphur Pluak Daeng and Ban Chang sub-dialects respectively. The seventh chapter is the comparative study of the phonological systems and the sounds of these Rayong sub-dialects. The last chapter is the conclusion and suggestion. The first appendix presents the word list used in the field work and the second is the sound spectrograms of Rayong sub-dialects.
