Infra-red absorption spectra of calcified tissues have been obtained in the region 5–15 μ, using potassium bromide and mineral oil as suspending media. A method of preparing the specimens as powders is described. It was found that whereas the absorption bands of a pure carbonate mineral (calcite) showed the expected inverse relation of absorption to particle size, no such relation was observed for bone, and only limited relation was observed with enamel. Thus at very small particle sizes the intensity of the v 3 and v 2 modes of carbonate in these substances was not consistent with the amount of CO 2 present. Studies of Na 2CO 3 in KBr as mechanical and fused mixtures revealed that in the mechanical mixture the out of plane mode near 11.5 μ ( v 2) consisted of a single peak, but in the fused mixture two separate peaks were seen indicating that the CO 3 −− ion was present in two different environments. When powdered enamel, dentine and bone were examined in the same region using high resolution, it was found that two absorption bands were also present, one at 11.38 μ and one at 11.45 μ. Annealing studies of enamel have revealed that three bands apparently arise from CO stretching modes of the carbonate ion at 6.5, 6.9 and 7.1 μ. During the annealing process the bands at 6.5, 6.9 and 11.38 μ become more intense. In view of the known vibrational characteristics of the carbonate ion these findings lead to the conclusion that in the calcified tissues this ion is present in two nonequivalent sites.