Thermal diffusivity (α) of amorphous silicon carbon nitride (a-SiCxNy) thin films on crystalline silicon, prepared by ion beam sputtering, has been studied using the traveling wave technique. The variation of thermal diffusivity with carbon content in amorphous silicon carbon nitride samples are reported. Thermal diffusivity decreased from ∼0.35 cm2/s for samples with carbon contents of less than 30 at. %, to about 0.15 cm2/s for a-SiCxNy films with a carbon content of ∼70 at. %. A similar variation was found for the film density as measured by surface acoustic wave spectroscopy as a function of the carbon content. The results on a-SiCxNy, elucidate the relation between thermal diffusivity and the bonding configuration, density and microstructure of the network. © 2001 American Institute of Physics.