This study presents the carbon, oxygen, and magnesium isotope compositions of two modern brachiopods, Terebratalia transversa and Frieleia halli, and one fossil specimen (2.3 Ma), Terebratula scillae. The aim of this study is to investigate the variability of these isotopic compositions and to evaluate the potential of brachiopods as a proxy of past seawater delta Mg-26 values. The two investigated brachiopod shells present the same range of delta(26)Mgvariation (up to 2%). This variation cannot be ascribed to changes in environmental parameters (temperature or pH). As has been previously observed, the primary layer of calcite shows the largest degree of oxygen and carbon isotope disequilibrium relative to seawater. In contrast, the delta Mg-26 value of the primary layer is comparable to that of the secondary calcite layer value. In both T. scillae and T. transversa, negative trends are observable between magnesium isotopic compositions and oxygen and carbon isotopic compositions. These trends can be explained by kinetic effects linked to changes in growth rate during the brachiopod life. The innermost calcite layer of T. transversa is in isotopic equilibrium for both oxygen and magnesium and could therefore be the best target for reconstructing past delta Mg-26 values of seawater. (C) 2016 Elsevier B.V. All rights reserved.