The focus of this thesis was to study the chemotaxonomic relationship of selected southern Australian marine brown algae of the genera Cystophora and Sargassum. Consequently, this resulted in the isolation and structure elucidation of six new terpenoids from two southern Australian marine brown algae Cystophora moniliformis and Sargassum fallax together with 10 previously reported natural products. As a result of the re-isolation of these known secondary metabolites, updated and complete structural characterisation data could be provided for the first time for 7 of these compounds. Chemotaxonomic studies of Cystophora moniliformis resulted in the isolation of two new cyclic epimeric terpene diols moniliforminol A (3.25) and moniliforminol B (3.26), a new linear farnesyl acetone derivative (3.27) and the previously described terpenoids (3.19)-(3.24). This study also resulted in the first complete 2D NMR characterisation for compounds (3.21) to (3.24) as well as the first report of (3.24) occurring as a natural product. All structures were elucidated by detailed spectroscopic analysis with the relative configurations of (3.25) and (3.26) being established by selective 1D nOe NMR experiments. The proposed biosynthetic pathway for the above compounds has also been described. Chemical investigation of the Southern Australian marine brown alga Sargassum fallax resulted in the isolation of three new meroditerpenoids fallahydroquinone (4.8), fallaquinone (4.9) and fallachromenoic acid (4.10), together with the previously reported compounds sargaquinone (4.1) (isolated and identified in a mixture with sargaquinoic acid), sargahydroquinoic acid (4.2), sargaquinoic acid (4.3) and sargachromenol (4.11). As a result of this study the complete 2D NMR characterisation for sargahydroquinoic acid (4.2) and sargaquinoic acid (4.3) could also be reported for the first time. All structures were elucidated by detailed spectroscopic analysis. Sargahydroquinoic acid (4.2) and sargaquinoic acid (4.3) displayed moderate antitumour activity.