The mechanical properties of amorphous silicon carbonitride (a-SiC (x) N (y) ) films with various nitrogen content (y = 0-40 at.%) were investigated in situ at elevated temperatures up to 650 A degrees C in inert atmosphere. A SiC film was measured also at 700 A degrees C in air. The hardness and elastic modulus were evaluated using instrumented nanoindentation with thermally stable cubic boron nitride Berkovich indenter. Both the sample and the indenter were separately heated during the experiments to temperatures of 300, 500, and 650 A degrees C. Short duration high temperature creep tests (1200 s) of the films were also carried out. The results revealed that the room temperature hardness and elastic modulus deteriorate with the increase of the nitrogen content. Furthermore, the hardness of both the a-SiC and the a-SiCN films with lower nitrogen content at 300 A degrees C drops to approx. 77 % of the corresponding room temperature values, while it reduces to 69 % for the a-SiCN film with 40 at.% of nitrogen. Further increase of temperature is accompanied with minor reduction in hardness except for the a-SiCN film with highest nitrogen content, where the hardness decreases at a much faster rate. Upon heating up to 500 A degrees C, the elastic modulus of the a-SiCN film decreases, while it increases at 650 A degrees C due to the pronounced effect of short-range ordering. The steady-state creep rate increases at elevated temperatures and the a-SiC exhibits slower creep rates compared to the a-SiCN films. The value of the universal constant x = 7 relating the W (p)/W (t) and H/E (*) was established and its applicability was demonstrated. Analysis of the experimental indentation data suggests a theoretical limit of hardness to elastic modulus ratio of 0.143.