We have developed and field tested thermally-activated gas canisters for use as temperature sensors during modified in-situ (MIS) oil shale retorting. These instruments allow one to determine when known retort bed positions reach a predetermined temperature. From this information, the degree of flow uniformity through the bed can be determined. The main advantage of this concept is that the thermal sensors need no physical connection to their respective data acquisition instruments. The construction and emplacement of these sensors is quite simple and inexpensive. In brief, a metal canister is filled with compressed freon (or other easily detectable gas) and sealed with a temperature sensitive cap. This cap is designed to open at a specific temperature. The released gas is then detected by analyzing the retort offgas using conventional gas chromatographic methods. The canisters are emplaced in the retort during construction. For the field test described here, we simply lowered them downhole onto the rubble pile. They are ruggedly designed to survive subsequent blasting operations.