Abstract: Temperatures of 0.2.degree. K or lower are achieved by feeding 3He and 4He separately into a mixing chamber (5) in an enclosure (3) in which the temperature is held at around 2.degree. K. The endothermal dilution of 3He into 4He provides the required cold. The resulting mixture (M) passes out of the mixing chamber and the enclosure while cooling the incoming fluids by means of exchangers (1, 12, 4). To compensate for thermal losses, the mixture (M) also undergoes Joule-Thompson expansion (12) optionally followed by evaporation (13), preferably between about 1.5.degree. and 2.5.degree. K, and the resulting cold is used to lower the temperature of the incoming fluids from well above 4.degree. K to between 1.5.degree. and 2.5.degree. K, which is close to the temperature prevailing inside the enclosure (13) containing the coldest point (6) in the circuit.