The effect of a constant magnetic field on the rheological and paramagnetic properties of some paraffin-base crude oils was studied. An increase in the viscosity and activation energy of viscous flow for high-paraffin oils with an increased amount of neutral resins and their decrease for high-paraffin oils with an increased amount of acidic resins were observed after magnetic treatment. The association processes that occur in disperse petroleum systems after magnetic treatment and determine the change in the rheological properties of petroleum depend on the contents of paraffin hydrocarbons, resins, and asphaltenes. A change in the degree of association of molecules upon magnetic treatment, which is accompanied by an abrupt chan-c in the activation energy of viscous flow, is indicative of structural transformations taking place in the petroleum system. The magnetic field displaces the association-dissociation equilibrium in the disperse petroleum system and leads to various relaxation effects.