The variation of opto electronic parameters like the optical band-gap (Tauc gap E-opt and the isoabsorption gap E-04), and structural parameters like the Tauc slope parameter B, of the hydrogenated amorphous silicon carbon alloy films prepared by r.f. glow discharge decomposition of silane and acetylene as well as silane and ethylene, are discussed for various compositions and also for different annealing temperatures. We observe, for the first time, an interesting new correlation between (E-04-E-opt) and the Tauc slope parameter B of amorphous semiconductors.