We report measurements on the C-i-O-i 'C-centre' in Czochralski-grown Si1-xGex (xless than or equal to0.06), using the complementary techniques of photoluminescence (PL) and deep-level transient spectroscopy (DLTS). Both techniques show that the donor level of the C-centre in SiGe alloys shifts towards the valence band with increasing x. Unexpectedly, the shift rate of d(DeltaH(n))/dx = +550 meV detected using DLTS is found to be 1.6 times greater than that from PL measurements. Alloy broadening of the PL line and the DLTS signal are similar, and suggest that the C-centre is preferentially found in a Ge-rich environment. (C) 2003 Elsevier B.V. All rights reserved.