A critical survey is made of selected basic data on those aqueous solutions needed to model geothermal energy utilization. The data are useful in the design and construction of power plants and for direct use. The result of the survey is given as a current status of data. More emphasis is placed on the viscosity, thermal conductivity and density of sodium chloride solutions up to 350/sup 0/C and 50 MPa. An ideal data book for geothermal energy is described.