Nanocrystalline powders with average particle sizes of 20 to 40 nm are obtained via the high-energy milling of microcrystalline TaC powder. The phase composition, morphology, specific surface area, and particle size of nanopowders are analyzed by means of X-ray and laser diffraction, SEM, and BET. The experimental dependence of the concentration of oxygen in nanopowders on their specific surface area is obtained. Thermodynamic simulation and fractional gas analysis show that oxygen in TaC nanopowders is mainly in the chemisorbed state and forms 5–7 monolayers of the Ta2O5 oxide phase on the surfaces of particles, regardless of their size. It is established that in contrast to microcrystalline powders, the heating of TaC nanopowders can be accompanied by considerable decarbonization and a change in phase composition.