The band structure of GeC epilayers on Si(0 0 1) substrates has been investigated by Fourier transformation infrared spectroscopy, photoreflectance and spectroscopic ellipsometry measurements. The Ge1-xCx epilayers were grown by molecular beam epitaxy with the substitutional C content x from 0 to 0.025. The absorption coefficient a of these epilayers obeyed a square-root law for the photon energy hv, indicating an indirect band gap semiconductor. The fundamental band gap energy E-g and the interband transition-energy E-0 remarkably decreased with increasing x. Large band gap bowing was found out in the GeC epilayers and large bowing parameter of 13.1 eV was evaluated. The modification of the band structure by the incorporation of C atoms was occurred. (C) 2003 Elsevier Science B.V. All rights reserved.