Contact potential difference (CPD) measurements using a Kelvin probe coupled with synchrotron radiation are used to investigate the surface photovoltage (SPV) induced by synchrotron radiation at GaAs surfaces. A large and quasi-permanent SPV is found at surfaces of low-doped and low-temperature samples. SPV discharge mechanisms are investigated. The CPD technique is used to define conditions under which SPV is negligible, leading to reliable measurements of band bending at low temperature. Band bending measurements are reported for interfaces between metals and GaAs (100) surfaces.