The Bungku-Tolaki group of languages (Austronesian, Western Malayo-Polynesian) comprises fifteen languages spoken in and around the southeastern peninsula of Sulawesi Island in present-day Indonesia. Although there exist no written records for these languages prior to 1900, I apply the traditional methods of historical and comparative linguistics, as well as bring to bear more recent understandings regarding the nature of grammatical and semantic change, in order to develop a picture of their common ancestor language, Proto-Bungku-Tolaki. The dissertation has two parts. In part one, I reconstruct the sound system of Proto-Bungku-Tolaki, detailing both the innovations which distinguish it from its nearest identified ancestor, Proto-Malayo-Polynesian, along with the phonological changes which occurred in the various daughter languages, bringing us up to the present day. In the second part I focus on issues of transitivity including the grammaticalization of the preposition taken as a valence-changing applicative suffix, clause structure including relative clauses, and verbal inflection. Herein, Proto-Bungku-Tolaki is reconstructed as having three construction types which allowed the expression of both an agent and a patent, namely the active, the passive, and the antipassive. Nominative and absolutive pronoun sets served as agreement markers, though the genitive subject marking original to subordinate temporal adverbial clauses has in some languages also made its way into main clauses. Because there is not as yet a significant body of published material on the Bungku-Tolaki languages, I have made a conscious effort to amply supply this dissertation with the primary data upon which my analyses have been based. Therefore although the present work is of particular relevance to Austronesianists working in the field of historical reconstruction, the data and descriptions alone should make this an invaluable reference for anyone interested in the languages of this small corner of the world. Appendices include five texts with interlinear glossing and free translation, and a compilation of Proto-Bungku-Tolaki lexical reconstructions with supporting evidence.