Ancient ivory, from the Chengdu Jinsha and Guanghan Sanxingdui sites in China, has been buried for several thousand years. In order to determine the degradation mechanisms and to provide a scientific basis for protecting them, these ancient ivory samples have been compared with modern ivory using infrared spectroscopy in the frequency range 400–4000 cm−1. By combining chemical analysis data we compare the crystallinity and crystal chemistry of the apatite component, as well as the structural characteristics of the ivory. These investigations showed that the ancient ivory consists almost entirely of hydroxyl-carbonate apatite as the predominant phase. Compared with the modern ivory, the PO43− and CO32− bands are stronger, the PO4RF values are obviously greater, and an extra OH− band at 3569 cm−1 is observed in the ancient ivory. These results indicate that there is a greater degree of apatite crystallinity in the ancient ivory and also imply that there has been incorporation and recrystallization of CO32− in the apatite during burial. Positive correlations have been found between the apatite crystallinity, CO32− and OH− ion contents, and burial time. The organic matter in ancient ivory has been lost or decomposed as the organic bands (e.g. at 1238 cm−1 and 1337 cm−1) have disappeared. This may be the main reason that ancient ivory is easily dewatered and readily friable after being unearthed.