When Z1 << Z2, inner-shell ionization of a target atom of atomic number Z2 by a projectile of atomic number Z1 occurs predominately via removal of an inner-shell electron to the target atom continuum (direct ionization). Electron capture contributes then insignificantly to the ionization, and thus the predictions of perturbative-in-Z1/Z2 theories of direct ionization can be tested through comparison with measured ionization cross sections. We present such a comparison with the recently reported data1 for K-shell ionization of the Z2 = 22, 26, 28, and 30 elements by 60–150 keV protons (Z1=1). These ionization cross sections were inferred from x-ray production measurements using Krause’s fluorescence yields.2