NOVELTY - Liquefying hydrogen feed stream containing both ortho and para species of hydrogen involves catalytically converting ortho species including desorbed ortho species to para species at high temperature; adsorbing ortho species to produce para-rich stream containing unadsorbed ortho species; introducing desorbed ortho species into higher temperature catalytic converter; introducing para-rich stream into lower temperature catalytic converter and converting unadsorbed ortho species to para species to produce ortho-lean stream; and preparing liquid hydrogen product stream from ortho-lean stream.    USE - Method for liquefying hydrogen feed stream containing both ortho and para species of hydrogen (claimed).    ADVANTAGE - The method reduces the amount of refrigeration and the amount of energy and cost involved in generating refrigeration.    DETAILED DESCRIPTION - Liquefying hydrogen feed stream containing both ortho and para species of hydrogen involves    (A) liquefying hydrogen contained in feed stream in hydrogen liquefier to produce liquid hydrogen product stream, in which the liquefier includes heat exchangers operating at successively lower temperatures to cool hydrogen;    (B) catalytically converting portion of ortho species including desorbed ortho species to para species in higher temperature catalytic converter;    (C) adsorbing further portion of ortho species in adsorbent of adsorption unit(s) to produce para-rich stream containing unadsorbed ortho species and regenerating the adsorbent in adsorption unit by desorption to produce desorbed ortho species;    (D) introducing desorbed ortho species into higher temperature catalytic converter;    (E) introducing para-rich stream into lower temperature catalytic converter and catalytically converting the unadsorbed ortho species to para species to produce ortho-lean stream having reduced content of ortho species as compared with the feed stream; and    (F) preparing liquid hydrogen product stream from ortho-lean stream; in which the higher temperature catalytic converter, lower temperature catalytic converter and adsorption unit are connected with the heat exchangers so that the higher temperature catalytic converter operates at higher temperature level greater than the lower temperature catalytic converter, and the adsorption unit operates at temperature not greater than that of the higher temperature catalytic converter and greater than that of the lower temperature catalytic converter.    An INDEPENDENT CLAIM is included for a liquefier comprising a compression system (12) for compressing hydrogen contained in feed stream (10); heat exchangers serially connected to one another and to the compression system for cooling hydrogen in feed stream; a refrigeration system connected to the heat exchangers for supplying refrigeration requirement to the heat exchangers so that the heat exchangers operate at successively lower temperatures; a higher temperature catalytic converter (90); adsorption unit(s) (38) having adsorbent beds (40, 42) operating in an out-of phase cycle so that further portion of ortho species in feed stream is adsorbed and para-rich stream is produced, while another adsorbent bed is undergoing regeneration and desorbing ortho species to produce desorbed ortho species; lower temperature catalytic converter(s) (50, 54); and an outlet connected with the lower temperature catalytic converter to discharge liquid hydrogen product stream comprising ortho-lean stream.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic process flow diagram of a liquefier.    Hydrogen feed stream (10)    Compression system (12)    Adsorption unit (38)    Adsorbent beds (40, 42)    Lower temperature catalytic converters (50, 54)    Higher temperature catalytic converter (90)