In this talk, we first briefly review the isospin dependence of the total nucleon effective mass M*J inferred from analyzing nucleon-nucleus scattering data within an isospin-dependent non-relativistic optical potential model, and the isospin dependence of the nucleon E-mass M*;EJ obtained from applying the Migdal–Luttinger theorem to a phenomenological single-nucleon momentum distribution in nuclei constrained by recent electron-nucleus scattering experiments. Combining information about the isospin dependence of both the nucleon total effective mass and E-mass, we then infer the isospin dependence of nucleon k-mass using the well-known relation M*J =M*;EJ . M*;kJ . Implications of the results on the nucleon mean free path in neutron-rich matter are discussed.