Abstract A thermodynamic framework is developed for calculating wax precipitation in multicomponent (nonferromagnetic) hydrocarbon mixtures under the influence of a static magnetic field. A mathematical expression was derived for calculating the compositional derivatives of the magnetic susceptibilities using an equation of state. These susceptibilities are required for calculating magneto‐chemical fugacities. The model was used to predict the effect of a static magnetic field on crystallization temperature and amount of precipitated wax for three normal‐paraffin mixtures and one synthetic gas‐condensate. Calculated results indicate that static magnetic fields increase the probability of formation of paraffin crystals, in accordance to the experimental evidence. © 2006 American Institute of Chemical Engineers AIChE J, 2006