The effect of a uniform magnetic field on the mechanism, kinetics, and resulting morphology of a spinodally decomposing alloy is investigated. It is shown that the magnetic field is most effective in forming uniformly spaced rods parallel to the magnetization if the aging is carried out just below the intersection (on the phase diagram) of the Curie temperature and the temperature where the spinodal instability sets in. This condition limits the effective magnetic aging alloys to the composition range near this intersection.