Described is a process and a device for liquefying hydrogen, whereby the hydrogen to be liquefied is liquefied against one or more refrigerant (mixture) circuits and/or by means of expansion and is guided over at least one ortho-para conversion catalyst. According to the invention, the ortho-para conversion catalyst or at least one of the ortho-para conversion catalysts (K) can be bypassed. Furthermore, during the liquefaction process an exchange (7, 8) can take place between the hydrogen fraction (3), flowing (2, 4) through the ortho-para conversion catalyst(s) (K), and the hydrogen fraction (3), not flowing through the ortho-para conversion catalyst(s) (K).