The variation of the modal propagation loss of planar SiGe/Si heterojunction waveguides with Ge concentrations ranging from 1.3% to 10% has been determined for both TE and TM polarizations at wavelengths of 1.15 and 1.523 &mgr;m. The results show that at 1.15 &mgr;m wavelength the propagation loss increases with increasing Ge concentration due to the band‐edge absorption, which dominates the waveguide loss characteristics, while at the wavelength of 1.523 &mgr;m it decreases with increasing Ge concentration. The polarization sensitivity is only found at the longer wavelength and is thought to be due to the interaction of the evanescent field at the SiGe/Si interface with the Si substrate. ©1995 American Institute of Physics.