Silicon thermal oxidation was used to characterize the interaction of Si interstitials with a Si1−xGex layer and with B in Si1−xGex. Diffusion of B in a Si marker layer, positioned under a Si1−xGex layer, was monitored as excess Si interstitials were injected into the bulk by the oxidation of the Si capping layer (T=850 °C, t=18 min). The Si1−xGex layer did not affect the oxidation‐enhanced diffusion of B in the Si marker layer. Hence, Si1−xGex (x<0.30) does not appear to be a strong sink for Si interstitials. In addition, the enhancement from Si thermal oxidation (T=800 °C, t=60 min) of the measured B diffusivity in Si1−xGex is similar to that in Si. This indicates that, as in Si, the mechanism for B diffusion in Si1−xGex (x<0.18) primarily involves Si interstitials. © 1995 American Institute of Physics.