Measurements are reported of the change in the thermal conductivity coefficients {up triangle, open}λ∥ and {up triangle, open}λ⊥ of polar gases under the influence of a magnetic field at room temperature. Investigated are the linear molecules HCl, DCl, HCN, OCS and CO2 and the symmetric top molecules CH3F, CD3F, CH3CN, CHF3, NH3 and ND3. The results on the nonequilibrium polarizations WJ and W{square cup}JJ obtained from these experiment are expressed in terms of effective cross sections. With the present results the amount of data now available on certain effective cross sections has grown to an extent that justifies a systematic overview. Correlations are presented between the effective cross sections and various molecular properties such as the electric dipole moment and the moments of inertia. © 1979.