Absolute cross sections for K ionization in thin foils of Zr, Sn, W and Pb by negaton and positon impact have been measured with the aid of a β-ray spectrometer and a NaI(Tl) scintillation crystal. The negaton or positon energies have been varied from 240 keV to 1440 keV. In all cases the values obtained are much larger than those given by the non-relativistic theory. No systematic difference has been found between the values measured with negatons and positons, respectively. According to the measurements the exponent of Z in the ionization cross section varies from ν = 2.60±0.20 to ν = 2.00±0.30 for electron energies between 240 keV and 1440 keV.