The history of the research program on thermophysical properties of refrigerants at the National Institute of Standards and Technology (NIST) is described in this paper. The program has roots extending back to at least 1909. The current program began in 1981 as a collaboration between the Physical and Chemical Properties Division and the NIST Thermal Machinery Group to provide properties for the refrigerant mixtures being investigated for use in heat pumps. With the implication of the CFC refrigerants in the destruction of stratospheric ozone, the program increased dramatically in scope in 1987. The program includes experimental measurements and modeling of both the thermodynamic and transport properties of a wide variety of fluids and fluid mixtures; these results are transferred to industry in a variety of ways, including a computer database known as REFPROP. Because of its long-standing program in fluid properties, NIST was in a unique position to respond to the urgent international need for data on new refrigerants.