Si/SiC- and Si/SiGeC-multiple quantum well (MQW) structures of different well thicknesses and C-concentrations up to 2.7% have been grown pseudomorphically on Si(001) using molecular beam epitaxy. Near bandedge photoluminescence (PL) is observed for Si/SiC for certain,growth parameters. Substrate temperature and Si growth rate strongly influence the structural and optical properties of the samples, in particular for samples with high C-content. The thermal stability of the Si/SiC-layers was investigated by using FL, high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). At anneal temperatures of 950 degrees C and below the structures relax only by interdiffusion but not by defect or SiC-formation. PL data on a nearly strainless Si/SiGeC-MQW indicate a band alignment of type-I character.