This work describes the isolation and structural elucidation of extractives from four species of the Meliaceae and one of the Simaroubaceae families. All five species examined are endemic to the island of Madagascar. One novel seco-ring A protolimonoid with a bourjotinolone A-type side-chain was isolated from Turraea sericea, while Malleastrum antsingyense yielded one known and one novel limonoid of the vilasinin group. Neobeguea leandreana was found to contain three novel limonoids of the phragmalin class, including a relatively rare 17-keto seco-ring D compound and one containing a oxidized C-19 methyl group. Quivisia papinae has afforded eight novel and five known protolimonoids and limonoids of the azadiradione, evodulone, and mexicanolide classes. Included among these are a mexicanolide group limonoid with a 17-keto seco-ring D, and two further mexicanolide limonoids containing a hitherto unreported 9a,11a-epoxide ring and a Δ 9(11)-double bond. One C19 and four C20 quassinoids, of which one is novel, together with a known but rare triterpenoid, were isolated from the Madagascan Simaroubaceae Samadera madagascariensis. These findings support the suggestion that this species is closely related to, if not synonymous with, the companion species Samadera indica and Quassia indica. A literature survey on the effect of structural variations in ring B on coupling constants in 11,12-disubstituted havanensin-group limonoids was also undertaken, resulting in the observation of a remarkable correlation between ring structure and coupling constant values for a wide range of compounds isolated from different sources. An explanation for these observations is advanced. / Thesis (Ph.D.)-University of Natal, Durban, 2001.