Collisions between an atom and a diatomic molecule are theoretically investigated. The detailed calculations are carried out in the special case of hydrogen gas at lower temperatures in order to interpret the observed relation between the viscosity of the gas and the para-ortho concentration ratio. It is found that, by improving the shape of the intermolecular potential, one can avoid the greater part of the discrepancy between the experimental fact and the result of an older theory. However, quantitative explanation of the relation requires more detailed and exhaustive calculations.