Astronomical studies of the CH and OH masers have been based on the hypotheses of both radiative and collisional pumping. The mechanism of collisional inversion of the ground state Λ-doublets by neutral perturbers was first described by Bertojo et al. (1976) who made a semi-quantitative analysis of the collision process, based on calculations of potential energy curves. More recent work (Kaplan and Shapiro, 1979; Dixon and Field, 1979; Flower, 1980) has, however, been critical of some of the assumptions made by Bertojo et al. Consideration of the symmetry and the form of the interaction of CH and OH with the spherically symmetric perturbers para-H2 and He shows that current knowledge of the interaction potential is insufficient for quantitative results to be obtained.