The dependences of absolute differential bremsstrahlung cross sections on 0.4–2-keV electron scattering by supersonic gas jets of Ar, Kr, and Xe versus electron energy as well as the atom number were studied experimentally. The bremsstrahlung photons have been detected at an angle to the incident electron beam in the ultrasoft x-ray spectral region 70–200 eV. It is found that as the electron energy is increased from 0.4 to 2 keV, the absolute bremsstrahlung cross section for Xe increases throughout the whole electron energy range and the energy dependencies of cross sections for Ar and Kr displayed maxima at electron energies of (Ar) and (Kr). These experimental results cannot be explained in the framework of Bethe and Heitler and Sauter theory, based on the first Born approximation. The experimental absolute differential bremsstrahlung cross sections for Ar, Kr, and Xe are considerably larger then the tabulated values of ordinary bremsstrahlung. The obtained dependence of bremsstrahlung cross section differs with calculated one, particularly for large atomic number .