The formation of molecular CO 2 in synthetic apatites (prepared in aqueous systems), dental enamel, dentine, and various apatitic rock phosphates after heating in the range 120-900°C has been investigated by infrared spectroscopy. The CO 2 band at 2340 cm -1 was observed in the synthetic samples, enamel, and some of the rock phosphates, but not in dentine or bone. It is suggested that the absence of this band in dentine and bone is caused by the small crystal size of their apatites. The CO 2 band at 667 cm -1 was never observed. The polarized infrared spectrum of heated dental enamel showed that the linear CO 2 molecules were either randomly oriented or oriented so that the length of the molecules made an angle of about 56° with the c axis. It is suggested that, if the latter is correct, the CO 2 originates from a CO 2-3 ion which occupied the sloping face of the phosphate ion site.