This dissertation presents a reconstruction of the phoneme inventory of Proto-Hlai, based on data from twelve Hlai languages spoken on Hainan, China. A classification of the Hlai languages is given with the innovations upon which it based, followed by a discussion of contact relationships and a discussion of reconstruction methodology. The inventory of Proto-Hlai initials is reconstructed, and original sesquisyllabic forms are shown to be necessary to account for the reflexes between the daughter languages; the initial inventory is also marked by the presence of aspiration on most consonants in word-initial position. This is followed by the reconstruction of the rime inventory, an outstanding features of which is two laryngeal components which are argued to have been the precursors to two of the synchronic tone categories in the daughter languages, and which conditioned segmental variation in most of the daughter languages. A comparison is made between Proto-Hlai, Proto-Be, and Proto-Southwest Tai, and a preliminary reconstruction of Proto-Southern Kra-Dai (the immediate ancestor of Proto-Hlai) is performed. When this reconstruction is compared with that of Proto-Hlai, it is shown that several important sound changes occurred in Pre-Hlai, including intervocalic obstruent lenition, vocalic transfer, aspiration of word-initial consonants, and peripheral vowel raising. The language Jiamao is examined in detail, and it is argued that Jiamao is a non-Hlai language which has been in close contact with Hlai since the Pre-Hlai period. An examination of the correspondences between Jiamao and Hlai reveal at least two layers of Hlai loanwords in Jiamao, and evidence Jiamao was originally very different from Hlai structurally. Finally, the Proto-Hlai lexicon is compared with those of other Southeast Asian language phyla, and it is shown that Hlai retains evidence of shared lexicon (via either a genetic or contact relationship) with Sino-Tibetan, Mon-Khmer, Hmong-Mien, and Austronesian, the last of which is particularly striking. The dissertation concludes with a summary of findings, empirical and theoretical contributions, and suggestions for future research.