No D2 was observed, and the calibrated intensity (see "Methods") of the weak C2D4 desorption near 200 K was ~2 x 10 13 C2D4/cm 2 , or just over 1% of a C2D4 monolayer. Thus, the alumina support had a small density of sites capable of binding ethylene at 150 K, but these molecules simply desorbed intact below 250 K. No change was observed in repeated TPD runs.