Photon-stimulated gas desorption yields from metal surfaces, typically used in large vacuum systems, such as stainless steel, copper, and electrodeposited copper, have been measured using synchrotron radiation with critical energies from 12.4 to 284 eV, thus allowing to obtain initial yields applicable to future large hadron colliders. The main gases desorbed from all surfaces are : H-2, CO2, CO, and CH4. Water is also desorbed and greatly increases with exposure until a saturation value is reached. In situ bakeout reduces the amount of water desorbed, but only slightly reduces the yield of the other gases. The dependance of the yields on critical energy is almost linear, except for H2O. Photoelectron yields have also been measured, showing a good linear relation with the pressure increase, thus suggesting a process where a photon produces an electron that subsequently desorbs a gas molecule.