Abstract Ab initio calculations are presented on the inversion barriers in mono-substituted amines and phosphines (NH2X and PH2X) and their radical analogs. As expected, using MP3/6-31G* energies, phosphine barriers are rather larger than those for the corresponding amines, and electronegative substituents increase the barriers. Cyano substitution causes a dramatic decrease in the ammonia barrier. All of the radical nitrogen species studied are predicted to have small or zero barriers, and the radical NH2O is predicted to be planar. PH2O also has a very small barrier.