The response of Kodak fine grain positive and Kodak no‐screen (duplitized) film was measured as a function of x‐ray energy over the range of approximately 4–52 keV under a vacuum of 10−3−10−4Torr. The x‐ray transmission properties were also determined. The calibrations were performed with fluorescence radiation excited by the bremsstrahlung from dc x‐ray machines. These two films can cover an exposure range of over three orders of magnitude. Some films, such as fine grain positive, exhibit an enhanced sensitivity if exposed under vacuum. Also, it was found that no‐screen film exhibits a sensitivity reversal at approximately 5 keV. Other films would be expected to show a similar response with the energy at which the reversal occurs determined by the characteristics of the emulsion.