A reaction path Hamiltonian is constructed that is based on a straight-line, least motion path that interpolates linearly between equilibrium reactant and product geometries of the molecular system. Conservation of linear and angular momentum are correctly accounted for. The resulting Hamiltonian has a Cartesian-type kinetic energy, the Coriolis coupling terms originally present in the kinetic energy having been transformed to potential energy coupling (hence the term ‘‘diabatic’’ reaction path Hamiltonian). Curvature coupling terms that appear in the original reaction path Hamiltonian, which is based on the minimum energy reaction path, are absent here because the present reaction path is straight. This new, diabatic reaction path Hamiltonian should be especially useful for describing H-atom transfer reactions in polyatomic systems, a case for which the minimum energy reaction path provides a poor description.