Twenty selenium barrier layer cells, made by a modern commercial process with a variety of modifications, were examined with irradiation of x-rays in the 15-40 kev energy range and the behaviour was compared with their response to light. The x-ray sensitivity is found to be independent of intensity of irradiation at constant x-ray tube anode voltage, but linearly proportional to anode voltage at constant anode current. The form of transient response to x-irradiation depends on the intensity of irradiation and the previous history of cell irradiation, a time of rest of about 2 hours being required to achieve the `first-irradiation' response. The fatigue effect does not occur in the steady-state values to photovoltage and photocurrent. There is some evidence for a qualitative relationship between x-ray and optical photovoltage sensitivities of a cell, but there is no relationship between the photocurrent sensitivities.