Abstract The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN x and a-CN x :H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN x :H films, was observed on the CTE, which reaches about ∼9 × 10 −6 C −1, close to that of graphite (∼8 × 10 −6 C −1) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10 −6 C −1 for 33 at.% nitrogen.