Mass absorption coefficient data published by Allen, Bragg, and Victoreen for the wavelength region 0.1 to 50 A are plotted for several common x‐ray tube and counter windows (Al, Be, and mica) and gases (Xe, Kr, A, and Ne) used in x‐ray detectors. A graph of the calculated percentage transmission of the windows is presented. The calculated quantum‐counting efficiencies for the range 0.1 to 15 A are shown for an end‐window argon‐filled Geiger counter, side‐window Xe, Kr, A, and Ne proportional counters and an NaI·Tl scintillation counter. Some of the factors to be considered in the choice of detectors for x‐ray diffractometry and x‐ray spectroscopy are discussed particularly with respect to the spectral sensitivity.