Optical pumping experiments have yielded very large disorientation cross sections (40–100 Å2) for collisions between oriented 2P1/2 atoms of potassium, rubidium, cesium and thalium, and helium, neon, argon, krypton, and xenon atoms in their ground states, in contrast to the extraordinarily small cross section (< 10−5 Å2) for the depolarization of 2S1/2 atoms, as found by Gallagher and by Anderson and Ramsey. Franz, Leutert, and Shuey pointed out that, within the adiabatic model, the transition ψ(j = 1/2, m = 1/2) → ψ(j = 1/2, m = −1/2) is strongly forbidden in conformity with Kramer's theorem according to which it is impossible to split the two components ψ(j = 1/2, m = ± 1/2) in an electric field. In the present investigation it has been shown that the inclusion of an angular nonadiabatic operator (i.e. the operator responsible for Coriolis mixing of adiabatic electronic molecular states) makes possible such transitions. Calculations are presented of cross sections for the resulting depolarization induced in collisions between potassium, rubidium, and cesium, and inert gas atoms.