Four of the most abundant gorgonian species from the southwestern Cape waters, Eunicella papillosa (Esper, 1797), E. alba (Esper, 1797), E. tricoronata Velimirov, 1971 and Lophogorgia flamea (Ellis and Solander, 1786) were analysed for Ca and Mg by atomic absorption spectroscopy (AAS) and ethylenediaminetetraacetate (EDTA) titration. The total mineral content in the peripheral tissues, excluding the axial skeleton, expressed as the sum of CaCO 3 and MgCO 3 of dry matter was between 65.5 and 83.5%. The mineral content varied in different growth regions and all specimens showed a higher degree of mineralization at the base than at the branch tips. The MgCO 3 concentration varied with genus and species and was between 9 and 11 mol %. The variation of the MgCO 3 concentration within different growth regions of the same species was small and generally did not exceed 0.8 mol%. From the branch to the stem, CaCO 3 and total mineral content was found to increase. The CaCO3:MgCO 3 ratios in different growth regions of all species indicated that the composition of the mesoskeleton with regard to the relative concentration of CaCO 3 and MgCO 3 is constant throughout the animal. Mineralogically, the mesoskeleton consists of high magnesian calcite as identified by X-ray diffraction. MgCO 3 concentrations determined by the peak shift method and by AAS were in fair agreement. The MgCO 3 data in gorgonian samples from the cold Atlantic Ocean and the warmer Indian Ocean show a linear relationship between water temperature and MgCO 3 concentration already demonstrated by Chave (1954). However, our data were consistently lower by I to 2% than expected.