The in situ surface activation of unmodified CaCO3 nanoparticles by interaction with surfactant in aqueous media has been studied, and the impact of this on the foamability and foam stability of aqueous dispersions was assessed. Using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of surfactant, air-water surface tensions, and relevant contact angles, the mechanism of this activation was revealed. The results show that the non-surface-active CaCO3 nanoparticles cannot be surface activated by interaction with cationic or nonionic surfactants but can be surface activated by interaction with anionic surfactants such as SDS and AOT, leading to a synergistic effect in both foamability and foam stability. The electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic surfactant headgroups results in monolayer adsorption of the surfactant at the particle-water interface and transforms the particles from hydrophilic to partially hydrophobic such that particles become surface active and stabilize bubbles. SDS is a more efficient surfactant for this surface activation than AOT. Possible reasons for this difference are suggested.