Pseudomorphic Si1−x−yGexCyalloy layers on Si (100) with band‐edge photoluminescence and without defect‐related luminescence have been achieved. The photoluminescence was measured from 2 to 77 K and was used to make a direct measurement of the band gap shift as a function of strain reduction as C was added. Compared to the effect of just reducing Ge content, results show that as C is added, strain is reduced more efficiently than the band gap is increased. Furthermore, results imply that a fully strain‐compensated Si1−x−yGexCylayer on Si (100) would have a band gap much less than that of Si, and suggest that initial C incorporation reduces the band gap of relaxed, unstrained Si1−x−yGexCyalloys. ©1995 American Institute of Physics.