This report describes a method the Bureau of Mines devised for measuring the dielectric constant and loss tangent of low-loss minerals at microwave frequencies. Determination of these properties makes it possible to predict the effects of using microwave heating in potential processing applications. The method uses a test cell and a radio-frequency impedance analyzer with automated data acquisition and analysis. In this initial phase of microwave heating research, the Bureau measured dielectric constants and loss tangents at frequencies between 100 mhz and 1 ghz. The objective was to establish a reliable data base for use in predicting the effects of microwave heating on minerals. Comparison with previously measured data showed the Bureau's measurement method to be accurate to within +- 3 pct. This report describes the method and discusses the preliminary results of the Bureau's investigations. Typical comparative data are included. This report is based upon work done under an agreement between the University of Alabama and the Bureau of Mines.