The reaction pd → dϕpsp is studied within the Bethe–Salpeter formalism. Under special kinematical conditions (slow backward spectator proton psp and fast forward deuteron) relevant for forthcoming experiments, e.g. at COSY, the cross section and a set of polarization observables are shown to factorize in the contribution of the pure subprocess pn → dϕ and a contribution stemming from deuteron quantities and kinematical factors. This provides a theoretical basis for studying threshold-near processes at quasi-free neutrons.