The Lewis and Brønsted basic properties of a stoichiometric hydroxyapatite (HAp) were investigated by infrared spectroscopy following the adsorption and desorption processes of a Lewis acidic molecule, CO2, and a Brønsted acidic molecule, C2H2. CO2 interacts with basic OH– and O2– of PO43– groups to form hydrogenocarbonates and surface carbonates, respectively. It also generates surface type A carbonates and related water upon substitution of two neighboring structural OH– groups. Water modifies the basic properties of the HAp by decreasing the surface carbonatation and enhancing the formation of hydrogenocarbonates, and promotes the substitution ability of OH– by carbonates. Due to the affinity of HAp for carbonatation, the thermodesorption experiment of CO2 accounts for the thermal decomposition of bulk type A and B carbonates rather than for the lone surface basicity. As for the acetylene probe, three nondissociative adsorption modes of acetylene on the HAp surface are observed: a π complex interaction with acidic POH, an interaction with an acid–base (POH–OH) pair, and finally, a σ complex interaction with basic OH– that is the most stable upon desorption. There is no evidence of the involvement of basic O2– of PO43– in the interaction with acetylene. It is thus proposed that both acidic POH and basic OH– groups may play a determinant role in acid–base properties of hydroxyapatites.