In response to a request from the Department of Energy (DOE), the National Bureau of Standards (NBS), 1979, undertook a program to develop the technology to utilize High Frequency Electromagnetic Mapping (HFEM) techniques for evaluating rubblized oil shale in the cold retort state in the modified in situ process. This technology development should also be applicable for using HFEM techniques for diagnosing, monitoring, controlling and evaluating modified in situ retorts after they are ignited. This report covers the work performed from October 1980 to September 1981. It describes the baseline data work required to design a high-temperature sample holder and experiments for determining the EM properties of oil shale samples at elevated temperatures (200 to 500/sup 0/C). A theoretical approach is given for modelling oil shale retorts for electromagnetic sensing techniques by a spheroid with an average dielectric constant along with numerical results. Finally, the measurement results are given for the spent and raw shale samples that were obtained from portions of the ten half-core samples plus the results of the electromagnetic transmission measurements taken on oil shale samples at LETC in Laramie, Wyoming. The work was accomplished in separate phases which centered around four main technical activities that are documented in this final report as follows: baseline data; theoretical work; spent and raw shale samples; and transmission measurements.