To characterize bed structure and measure fluid dispersion under varied flow conditions, conventional tracer measurements in packed beds rely on detection of specific molecular species. The work reported here demonstrates the use of temperature sensors, such as thermocouples, to develop similar kinds of information from thermal data. In oil shale retorts (that is, packed beds in which the process is driven by heat transfer from a flowing medium to the rubble), much can be inferred about bed and void structure and flow patterns from the time-temperature relationships among thermocouple arrays placed in the bed. The use of temperature data as a bed diagnostic tool in oil shale retorting experiments is shown. For combustion retorting, the close relationships between the thermal effects produced by condensing steam and those produced by retorting are illustrated. This thermal logging technique has proved useful in understanding laboratory retorting experiments and predicting retort performance. 5 figures, 6 tables.