Although radiogenic free radicals are widely used as molecular probes in biological calcified tissues, their nature and their location remain controversial. In the present study we have synthesized and irradiated apatites in which ¹³CO₃^2- ions substitute hydroxyl or phosphate ions in hydroxyapatite. The ¹³C-couplings show that the carboncentered radicals are CO₂^-—rather than CO₃^3-—the locations of which depend on the type of carbonate and the treatment of the sample.