The influence of oxidation of a Si-cap layer on the thermal stability of Si/Si_0.863Ge_0.137/Si heterostructures is investigated by high resolution x-ray diffraction. The role of the surface modification is extracted by comparison of dry oxygen and inert (nitrogen) ambient anneals at 810 and 900°C. Anomalous strain relaxation was observed in the sample oxidized at 810°C. It is hypothesized that the effect of anomalous strain evolution deals with the enhanced surface diffusion and roughening at the Si/SiGe interface.