Abstract In order to explain the expanded a-axis of human enamel apatite (9.441 Å) compared to that of synthetic (prepared at 900 °C) or mineral (Holly Springs, Ga.) OH-apatite ( a = 9.422 A ̊ ), the incorporation of chloride in apatites prepared from aqueous systems at 70–100 °C was studied. These synthetic (Cl, OH)-apatites were prepared either by precipitation or by hydrolysis of monetite, CaHPO 4 in NaCl solutions. The incorporation of chloride into the apatite structure (Cl-for-OH substitution) is reflected in the expansion in the a-axis and contraction in the c-axis dimensions. In aqueous systems (100 °C) chloride incorporation is limited to 40 mole per cent (= 3 wt per cent) while from solid state systems (1000–1600 °C), 100 mole per cent (= 6.7 wt per cent Cl) substitution is possible. The chloride incorporation in aqueous systems is further limited when CO 3 or F is also present in solutions from which the (Cl, OH)-apatites form. Results from these studies indicate that chloride incorporation in biological apatites is very limited. The expansion of the a-axis dimension of human enamel apatite can be partially attributed to some Cl-for-OH substitution and the rest of the expansion must be explained by other manners of substitution (e.g. HPO 4-for-PO 4; H 4O for OH).