We report on band-gap luminescence in short period, strain symmetrized (Si)m(Ge)n superlattices grown on relaxed, step-graded Sil-xGex alloy buffer layers. The dislocation density in the superlattices, which were grown at 500°C using Sb as a surfactant, is reduced by 2-3 orders of magnitude compared with superlattices grown on thin, partly relaxed Sil-xGex buffer layers. Due to the improved quality of the superlattices, well defined band-gap luminescence could be observed which is for a (Si)6(Ge)4 superlattice strongly enhanced compared with a Si0.6Ge0.4 alloy reference sample. The measured band-gap energies compare well with theoretical predictions. To study the influence of interdiffusion of the Si- and Gelayers on the band-gap of the superlattices, the samples were annealed and studied with photoluminescence and Raman spectroscopy. An increasing band-gap and a decreasing luminescence efficiency was found with increasing intermixing of the layers. These experimental results are well described with an interdiffuision model of the layers in conjunction with an effective mass calculation.