An OFHC copper chamber was exposed to 3.75 keV critical energy synchrotron radiation for a total dose of of 2.7 x 10(23) photons/m. After this dose, photodesorption yields down to 10(-5) molecules/photon were reached for H-2, CO, and CO2 and 10(-7) mol/photon for CH4, H2O, and O2 and a total of about one monolayer of gas was desorbed. From observation of the transient behavior of the desorption at the beginning of an irradiation period after a prolonged interruption, the desorption yield was found to be proportional to the coverage and followed an exponential decay with increasing photon dose. This is in contrast with the normal behavior where the desorption yield decreases linearly with dose on a log-log scale. The observations are interpreted by a two-step process involving photons or photelectrons to create a new surface phase of molecules which subsequently desorb thermally. With die chamber heated to 70-degrees-C, the photodesorption yields for H2O and O2 increased but the others remained relatively independent of the temperature. At the end of the irradiation period a substantial wall pumping was observed in the chamber. For CO, CO2, and O2 it was about 1160, 280, and 80 l/s respectively.