Sintering compacts of carbonated hydroxyapatite (CHA) nanoparticles (3.4wt% CO 3 2− ) in a CO 2 flow (4mL/min) proceeded at a temperature which was more than 200°C lower than that for hydroxyapatite in air (1150°C). During heating from RT to 1200°C (5K/min) the rate of shrinkage of the CHA compacts showed a maximum thrice as high as that in air at about 929°C. The shrinkage correlates with a mass loss caused by the release of CO 2 due to the thermal decomposition of CO 3 2− ions that substitute PO 4 3− ions in the CHA lattice. Firing the compacts in the CO 2 flow at 800 and 900°C for 2h resulted in an additional carbonatation on the B-sites and a further decrease in the sintering temperature to 890°C. The compacts fired in the 900–1000°C range became almost complete ceramics with high densities and mechanical properties close to those of medical implants. Firing at temperatures above 1000°C resulted in an additional carbonatation on the A-sites. However, this led to a material with low densities and poor mechanical properties. A supposition has been proposed that the effect of CO 2 gas-activated sintering is a result of the intensification of the diffusion in the nanoparticles caused by CO 2 molecules entering the bulk from the CO 2 atmosphere and (or) releasing from the bulk due to the decomposition of carbonates on the B-sites in the lattice.