The L X-ray spectra of Dy and Au produced by proton impact have been measured over a broad range of projectile energies. From the intensities of characteristic lines the relative ionisation cross sections sigma _i of the three L subshells L_i were derived. The cross section ratios sigma ₁/ sigma ₂ and sigma ₁/ sigma ₃ vary significantly with projectile energy, attaining a maximum at low energies. This behaviour cannot be quantitatively described by available theoretical models.