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The International Temperature Scale of 1990, ITS-90, came into effect on 1 January 1990, replacing the International Practical Temperature Scale of 1968, IPTS-68, and the "Echelle Provisoire de Température de 1976 entre 0,5 K et 30 K", EPT-76. The present paper reviews the various experimental measurements of thermodynamic temperature T (often expressed as differences (T - T68)) in the range 0,65 K to 1 357,77 K, which were available prior to September 1989, and on the basis of which values of temperature were adopted in the ITS-90. Estimates are given for the uncertainties in the derivation of these values.

Gas-thermometer measurements between 4 K and 100 K have been performed at the Kamerlingh Onnes Laboratory as part of the preparatory work in several institutes for a future, revised, International Practical Temperature Scale. Gas-thermometer temperatures are compared, between 13.8 K and 100 K, with platinum resistance thermometers calibrated on IPTS-68 and, between 4.2 K and 27.1 K, with rhodium-iron resistance thermometers calibrated on the National Physical Laboratory (Teddington) Scale of 1975 (NPL-75).Results of the measurements, with NPL-75 at 20.3 K as a reference temperature for the gas thermometer, show agreement with NPL-75 within 0.3 mK over most of the range. Differences from NPL-75 of 0.6 mK were found in the range 10 K to 14 K and around 27 K.Below 50 K, IPTS-68 is shown to be 4 mK to 10 mK higher than thermodynamic temperatures. Between 70 K and 100 K, IPTS-68 is 9 mK lower than thermodynamic temperatures.