The present invention provides a method of liquefying a hydrogen feed stream and a liquefier for carrying out such a method in which ortho-species of hydrogen contained in a hydrogen feed stream is converted to the para-species in higher and lower temperature catalytic converters. An adsorption unit, located between the higher and lower temperature catalytic converters, adsorbs a portion of the ortho content of the feed stream. The adsorbed portion is desorbed during regeneration of an adsorbent bed of the adsorption unit and is recirculated back for treatment in the higher temperature catalytic converter to reduce the degree to which the ortho-species are converted to the para-species in the lower temperature catalytic converter and at lower temperatures.
