Amorphous wide bandgap semiconductor thin films of the pseudobinary compound (SiC)1-x(AlN)x were grown by radio frequency dual magnetron sputtering on CaF2, MgO and glass substrates. We performed isochronical annealing steps up to 500°C. The optical bandgap is determined for each composition from spectroscopic transmission measurement in two different ways: according to Tauc and using the (ahv)2 plot. The dependence of the optical bandgap on the composition x can be described by Vegard's empirical law for alloys. © (2010) Trans Tech Publications.