© 2018 American Chemical Society. Hydroxyapatite (HA) and carbonated apatite (CAp) constitute the mineral part of biological hard tissue and are key materials in dental implants and bone regeneration. This work provides a density functional theory study of the static and dynamic properties and reactivity toward water of the three families of HA and CAp surfaces, namely, the (001), (010), and (101) surfaces. For clean materials, surface energy, dipole moment across the slab and vibrational features of the CO32- group in CAp are compared with either the same surface property of HA, or with the bulk feature. The full substitution of OH- groups by CO32- in CAp affects the water adsorption features significantly compared to HA. When CO32- is not directly exposed at the surface ((001) and P-rich (010) surfaces), the water-binding energy is increased/decreased by less than 25% compared to the values for the corresponding HA surfaces. For the CAp Ca-rich (010) surface, in which the CO32- group is in direct contact with water, the binding energy shows a 50% increase. For both HA and CAp (010) stoichiometric surfaces, water is spontaneously dissociated with the resulting OH- ion shared between three surface Ca ions.