The interfaces in Si0.65Ge0.35/Si superlattices grown at different temperatures (250–750 °C) and number of repetitions (5, 10, 20) are studied by x-ray and Raman scattering techniques. At 250 °C the interfaces are chemically abrupt, but exhibit a pronounced vertically correlated physical roughness that increases in magnitude with thickness. In the range of 400–550 °C interfaces are smooth but intermixed over at least two monolayers, and asymmetrically broadened with the alloy/Si interfaces width about twice (0.25 vs 0.5 nm) that of the Si/alloy interfaces. At higher temperatures (620–750 °C) undulations of 1 nm amplitude and 100 nm length scale develop at the alloy/Si interfaces. This morphological change appears to be strain-driven and causes some relaxation of the built-in strain. Moderate annealing (20 s at 750 °C) induces partial relaxation of the superlattices and out-diffusion (∼0.5 nm) of Ge into the contiguous Si layers, but has relatively little effect on the interfacial widths.