The absolute x‐ray response of three X‐UV photodiodes was measured over an energy range of 1.5–17.5 keV so that they could be used to calibrate x‐ray imaging systems for the ASCA satellite mission. An intense electron‐beam x‐ray generator was used to test both the dc and ac x‐ray response at 1.5, 4.5, 8.0, and 17.5 keV, and an55Fe source was used to examine one of the photodiodes at 5.9 keV. The x‐ray response was determined by comparing the X‐UV diode signal to that of a previously calibrated silicon surface barrier diode (SBD). The X‐UV detector response was similar to the SBD response at low energies (1.5 and 4.5 keV). At 8 keV, the X‐UV detectors exhibited about 70% of the SBD response, and at 17.5 keV, about 50%. This result is surprising, because the X‐UV diodes actually have a greater silicon thickness than the SBD. In contrast to our findings for SBDs in the past, this implies that not the entire physical volume of these detectors comprises the active volume. The X‐UV detector x‐ray response was also examined as a function of the applied bias voltage. No significant bias voltage dependence of the x‐ray signal was found, which indicates that the depletion layer thickness does not determine the active volume either. However, the detector noise was found to decrease substantially as the applied bias was raised from zero to a few volts. Response of these detectors, operated in pulse mode, to226Ra &agr; particles indicated large charge carrier recombination in the bulk silicon. This feature renders these detectors unsuitable for most charged‐particle spectroscopy applications.