Total K-shell ionisation cross sections were measured for the irradiation of thin solid targets ( < 5 μg cm 2 ) of 92Mo, 104Pd, 120Sn, 138Ba, 140Ce, 1142Nd and 144Sm with protons of 0.5 MeV u ⩽ E Lab M Proj ⩽ 6.0 MeV u . Only isotopes were chosen as targets for which internal conversion of de-exciting transitions of nuclear excited states does not contribute to K-shell ionisation. The measured cross sections are compared with theoretical calculations within the framework of the perturbed-stationary state theory and with calculations based on the semiclassical approximation. Only for very low proton energies ( ξ ⩽ 0.25) are there systematic differences between the experimental data and the theoretical calculations; for ξ > 0.25 most of the measured ionisation cross sections agree with both calculations within the experimental errors (9–15%).