The measurement of the stopping powers and stragglings for 8–300 keV protons in C, Al and Au complete the series of measurements performed with heavy ions under identical conditions. Effective-charge fractions, unaffected by target-thickness uncertainties, are derived and compared to the current theory of Brandt and Kitagawa. The energy-loss stragglings of protons are in good agreement with the Bonderup-Hvelplund theory, whereas the energy-loss stragglings of heavy ions exceed the theoretical predictions. It confirms the influence of non-equilibrium regions of targets on the results of energy-loss experiments performed with slow ions which we observed before.