AbstractPentaerythritol is, for a number of given reasons, notable among the relatively limited choice of materials suitable for the manufacture of Bragg X‐ray analysers for use in quantitative X‐ray spectrometry to relatively long wavelengths. In many, perhaps most, Bragg X‐ray optical systems the beam power throughput to the detector, and hence the system sensitivity, is directly proportional to the Bragg reflection integral. Absolute beam power spectrometry thus requires knowledge of the wavelength function of that quantity for the analyser used. In this paper the nature of that function, and its stability, is investigated for diffraction from the (002) planes of pentaeryth