We present experimental results on Hall mobilities of electrons in tensile strained Si1-yCy layers with a substitutional carbon yS=0.4%, but different concentrations of interstitial carbon. Although the lattice distortion due to misfit strain and hence, the band alignment are identical for all investigated samples, we find differences in electron mobility of nearly a factor 2 due to the varying concentration of interstitial carbon. For the highest interstitial C concentration (1×1020 cm-3), it was not even possible to obtain any reliable electrical data. We demonstrate that it is not sufficient to consider only strain in evaluating electrical properties of C containing layers. Specific growth conditions can lead to very different electrical properties due to the different amounts of interstitial C, even for pseudomorphically strained layers with the same lattice mismatch and band alignment. © 2000 American Institute of Physics.