SiC_xN_y: H thin films with continuously variable compositions have been preparedby using the plasma-enhanced chemical vapor-deposition (PECVD) method. The composition of the films is analyzed by using FTIR and AES. The experimental results show that the ratio of N/(N+C)in the SiC_xN_y: H films can be obtained rapidly from the FTIR absorption spectrum. Analyses of films thermally annealed rapidly by using FTIR indicate that the passivated films, which have been prepared by PECVD, have good thermal stabilities.