This manuscript describes the use of the tapered element oscillating microbalance (TEOM) to monitor the formation of elemental carbon under extreme reaction conditions, specifically the steam reforming of methane at 650 degreesC and operating pressures as high as 200 psig. Since the TEOM reactor passes the entire feed stream over all of the catalyst bed under a variety of process conditions at high temperatures and high pressures, one can get an accurate assessment of the relative change in catalyst mass, and therefore, carbon deposition with time on stream. In particular, the studies clearly show the accelerated rate of carbon deposition on Ni catalysts which occurs at elevated pressures during steam methane reforming.