[en] The design is given of an economical H₂ and D₂ liquefier which works by the boiling off of liquid He vapour. The decisive role played by the gas condenser in the operation of the liquefier is demonstrated. The relationships between the flow rates of He and of the liquefied gas are calculated and measured. The helium consumption is 2.85 for liquefying 1 l of H₂ and 3.8 1 l of D₂, which agrees with the calculations. (author)