A study is presented of the effect of a magnetic field on the viscosity of HD, N2, CO and CH4 . The temperature dependence of this viscomagnetic effect has been measured in the range between 300 K and the boiling point of these gases. The results are presented in terms of effective cross sections which depend directly on the angle dependent part of the intermolecular potential. The cross sections for reorientation of angular momentum have been compared with those obtained from N.M.R. measurements.