The magnetic mechanism parahydrogen conversion, catalyzed by chromia, has been found to be accelerated by an extrinsic magnetic field. The effect may be observed over ruby (a dilute solid solution of chromia in α-alumina), over various surface concentrations of chromia supported on α-alumina, and over pure α-chromia. If the paramagnetic species is dilute (as in ruby) the acceleration becomes larger below room temperature. If the paramagnetic species is sufficiently concentrated to show appreciable exchange interaction, then the acceleration becomes a complicated function of the surface magnetic phases present. The acceleration tends toward saturation at magnetic fields approaching 10,000 Oe. © 1969, American Chemical Society. All rights reserved.