L-subshell ionization cross sections by proton bombardment (0.4–2.0 MeV) on Pt, Au, and Hg have been extracted from measured L X-ray yields. The cross sections are compared with the predictions of the plane-wave Born approximation (PWBA), the constrained binary-encounter approximation (CBEA), and the PWBA including binding energy and Coulomb deflection effects. The latter effects account for the gross quantitative discrepancies between the PWBA and the data for proton energies less than 1 MeV, but disagreements still exist.