Synchrotron radiation circular dichroism (SRCD) spectroscopy is emerging as an important new tool in structural molecular biology. Previously we had shown that in lower-flux SRCD instruments, such as UV1 at ISA and beamline 3.1 at the SRS, vacuum ultraviolet (VUV) radiation damage to proteins was not evident after exposure over a period of hours. No effects were detected in either the protein primary or the secondary structures. However, with the development of high-flux beamlines, such as CD12 at the SRS, this issue has been revisited because of changes observed in the SRCD spectra of consecutive scans of protein samples obtained on this high-flux beamline. Experiments have been designed to distinguish between two different possible mechanisms: (i) photoionization causing free radicals or secondary electrons producing degradation of the protein, and (ii) local heating of the sample resulting in protein denaturation. The latter appears to be the principal source of the signal deterioration.