Measurements of IR spectra and low energy absorption, using the constant photocurrent method, have been carried out for a-Si:H/a-SiCx:H superlattices fabricated by an r.f. plasma deposition technique. It has been found that there exist high concentrations of SI-H bonds and high concentration of Si-C bonds at a-Si:H-a-SiCx:H interfaces. The interfacial hydrogen shows a lower thermal stability than that of the hydrogen in the bulk materials. A possible microscopic mechanism of interfacial defects in the a-Si:H/a-SiCx:H superlattices is discussed.