The proton magnetic resonance (pmr) spectra of a series of aminophosphines, R2NPXY, have been examined over the temperature range 40 to –150°. In the majority of these compounds the nitrogen substituents, R, became diastereotopic at low temperatures. Using the technique of matching the observed and computer simulated line shapes, it was possible to calculate the rates of, and activation parameters for, the implied stereochemical changes. The observed steric deceleration with increasing steric bulk of the nitrogen substituents, together with the inability to observe a barrier in 2,2-dimethyl-1-diphenylphosphinoaziridine, provide evidence that the observed barriers in the acyclic aminophosphines relate to torsion around the phosphorus–nitrogen bond rather than to pyramidal nitrogen inversion. The origins of these barriers are discussed from the standpoints of steric effects, lone pair–lone pair repulsions, and pπ–dπ bonding. In contrast to an earlier report it is found that the symmetrical aminophosphines, R2NPX2, are still undergoing rapid P–N bond rotation on the nmr time scale at –80°. The observation of diastereotopic R groups below –120°, together with steric considerations, suggests that the gauche-type conformation is adopted at low temperatures. The pmr spectra of C6H5As(Cl)N(CH3)2 have also been recorded over a wide range of temperatures, leading to the measurement of the first arsenic–nitrogen rotational barrier.