This innovative study investigates 18 Turbinaria and Sargassum brown seaweed samples from three archipelagos of the South West Pacific Ocean. The phenolic content of crude and size-fractionated extracts was determined by the Folin-Ciocalteu method; fucoxanthin was measured using high performance liquid chromatography. Although the phenolic content proved to be low for the species of both genera tested, the levels in the species of Turbinaria were higher than those of Sargassum tested, except in the Fiji islands where both species produced similar contents. These investigations also highlighted variations of total phenolic content with spatial, reef morphology and depth. A large number of small phenolic compounds (<2000 Da) were observed irrespective of the genus analyzed. Nevertheless, the composition of the phenolic pool varied in relation with the genus, the archipelago from which the material was collected, geomorphological features and the depth of each collection field site. A similar observation was for fucoxanthin content and in general, the measured constituents of the Turbinaria were less than those of the Sargassum species. The results are discussed in terms of inter- and intra-specific variability. Variations in both phenolic and fucoxanthin content were noted in tropical members of the Sargassaceae; such differences could be a result of specific chemical defense mechanisms adopted by members of each genus.