The surface of a nanosized silicon carbonitride powder synthesized by chemical vapor condensation from hexamethyldisilazane was analyzed by FT-IR spectrometry. Several chemical species were identified, and their behavior toward probe molecules was studied. Deuterium and methanol additions brought evidence of open pores. A slight oxidation led to pore coalescence and strong modifications in the chemical composition of the first atomic layer even though a complete silica coverage was not reached. Moreover, the SiCN surface further nitrided by ammonia addition at high temperature appeared to have, under those conditions, a behavior different from the silicon carbide surface.