Intermediate field theory is used to obtain the matrix elements which determine the action of a homogeneous magnetic field on the energy levels of triplet states of asymmetric rotor molecules. Applications of these formulas are discussed (i) in relation to the Zeeman effect on the rotational fine structure of triplet–singlet transitions, where conditions are identified under which individual lines remain unbroadened by the field, and (ii) in connection with the magnetic tuning of singlet–triplet resonance.