Effective refractive-indices of both TE and TM modes of passive semiconductor slab waveguides in the wavelength range near half the bandgap are measured and modeled, Comparisons among a bulk waveguide and several multiple-quantum-well (MQW) waveguides show that the;layered structures of a MQW tend to enhance the birefringence, Also, for a waveguide there exists a birefringence maximum at a wavelength that depends on the guiding layer thickness, Theoretical calculations with a model of anisotropic guiding layers for MQW waveguides show good agreement with the experimental results.