The spin-orbit coupled ground states of Kr+. and Xe+. (RG P-2(1/2) and RG P-2(3/2) RG=rare gas) are separated on the basis of ion mobility. Ion mobility separation of Kr+. and Xe+. is based on different interaction potentials of the P-2(1/2) and P-2(3/2), ionic states with helium. The ion mobility instrument used for these studies is a novel instrument based on a Fourier-transform ion cyclotron resonance mass spectrometer. The assignment of the spin-orbit coupled states of Xe+. is consistent with results from charge exchange ion-molecule reactions of Xe+. with N2O. (C) 2001 American Institute of Physics.