Recent accurate thermodynamic data from a number of laboratories are combined to provide the basis for a reference function for platinum resistance thermometer interpolation between 13.81 K and 273.15 K. The values of thermodynamic temperature for the defining fixed points of the International Practical Temperature Scale and also for a number of secondary points are more accurately assigned than heretofore. It is concluded that a temperature scale based on this reference function and these more accurate values of the defining fixed points would represent thermodynamic temperatures to within ± 2 mK. The reference function and the new values of the defining fixed points are proposed for use in the revision of the International Practical Temperature Scale.