Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca-10(PO4)(6)(F,Cl,OH)(2) solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for Cl-37/Cl-35 homogeneity using SIMS followed by delta Cl-37 characterisation by gas source mass spectrometry using both dual-inlet and continuous-flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than +/- 0.10% (1s) for the five samples with > 0.5% m/m Cl and +/- 0.19% (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38% between the mean Cl-37/Cl-35 ratios measured on three oriented apatite fragments. Furthermore, the results of GS-IRMS analyses show small but systematic offset of delta Cl-37(SMOC) values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 10(3)delta Cl-37(SMOC) values between +0.09 and +0.42 and in all cases with 1s <= +/- 0.25.