Deflection of3S1and3P2metastable rare‐gas atoms in an electron‐impact‐excited thermal supersonic expansion by an inhomogeneous magnetic field produces well‐defined daughter beams for eachMstate. The deflection patterns are spatially resolved with a translatable detector and used to derive fine‐structure populations in the3P0and3P2states, and to measure absolute efficiency of state selection by optical depletion. The fine‐structure population ratio3P2/3P0is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but theM‐state‐resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.