We have calculated mean escape depths (MEDs, D) of photoelectrons from Si, Cu, and Au excited by linearly polarized X-rays over the 50–10,000eV energy range for high-energy photoelectron spectroscopy. These calculations were done with Monte Carlo methods using the values of electron inelastic mean free paths λ that were calculated by the relativistic full Penn algorithm and the Dirac–Hartree–Fock atomic potential for elastic scattering of electrons. In order to know the dependence of MED on the asymmetry parameter β and the emission angle θ, we calculated MEDs for β=−1 to β=2 (by 0.5 steps) and θ=0° to θ=80° (with respect to the surface normal). The resulting MED values increase as the β values decrease over the 50–10,000eV energy range. We also found that the values of D/λcosθ are approximately constant to within 10% for a given value of θ and for energies over 1000eV (except for the condition of β=−1) over a range of emission angles (typically 0–40° or 60°). We also found that the energy dependence of the ratios D/λcosθ at emission angle θ=0° can be expressed by the Jablonski–Powell predictive equation over the 50–10,000eV energy range.Finally, we have obtained a predictive MED equation that could be applied to the wide range of experimental conditions that are used in high-energy XPS with synchrotron radiation.