We present chlorine stable isotope compositions of high temperature hydrothermal vent fluids from two mid-oceanic ridges. The six hydrothermal fluids come from sites along the slow-spreading Mid-Atlantic Ridge (Menez Gwen at 37 degrees 50' N, Lucky Strike at 37 degrees 17' N and Rainbow at 36 degrees 14' N) and the ultrafast-spreading East Pacific Rise (from three contiguous segments centered, respectively, at 17 degrees 25' S, 18 degrees 15' S and 18 degrees 26' S). These samples exhibit a large range in temperature, pH, chlorinity and Mg content reflecting various geological settings and substrata nature (basaltic or ultramafic). Although showing a large range of chlorinity from 251 to 821 mmol/kg, the fluids display homogeneous chlorine isotope signatures: delta(37)Cl(SMOC)= -0.03 +/- 0.06 parts per thousand, indistinguishable from that of seawater. This homogeneity contrasts with the large range of delta(37)Cl previously observed in continental or off-axis oceanic fluids. This study explores possible processes that may change Cl concentration during fluid genesis and discusses their theoretical isotopic effects. The delta(37)Cl homogeneity implies that, at the P-T conditions of the ridge axis, the process of phase separation (at sub- or super-critical conditions) that explains most of chlorinity variations, does not significantly fractionate chlorine isotopes. This study shows that the physical and chemical processes that likely alter Cl concentrations in ridge axis systems (i.e. transport processes, mixing or water-rock interaction) contribute to minimize the range of delta(37)Cl around the seawater delta(37)Cl value. (c) 2005 Elsevier B.V. All rights reserved.