The 21 TPD run experiments took >20 hours each, making repetition impractical and raising the possibility that surface contamination might have influenced the results in later cycles. Therefore, we also carried out repeated experiments studying the effects of the first six TPD runs, which, indeed, were responsible for most of the change in catalyst properties. Quantitative analysis of the desorption is based on these six-TPD experiments. As described elsewhere it is possible to calibrate the absolute sensitivity of the TPD system, and Table gives the numbers of C2D4 and D2 molecules desorbing per Pt4 cluster in each of the 6 TPD runs, averaged over the four available data sets. Because there was no evidence of adsorbed hydrogen remaining at 750 K, the number of C atoms deposited should be equal to the number of D2 molecules desorbing, and the total number C2D4 molecules adsorbed during each 150 K dose can be estimated as the number C2D4 desorbing + half the number of D2 desorbing. During the 1 st TPD, an average of ~2.9 C2D4/Pt4 were adsorbed, of which ~56% desorbed intact, with the remainder dehydrogenating to liberate D2 and deposited an average of ~2.5 C atoms/cluster. Because of the cluster isomer distribution, some cluster-to-cluster variation is expected, but we interpret the 1 st TPD desorption numbers as indication that three C2D4 molecules typically adsorbed per cluster at 150 K, two of which typically desorbed intact upon heating, the other one decomposed to liberate 2 D2(gas) and deposited 2 C per cluster, on average. By the 6 th TPD cycle, the number of C2D4 adsorbed per Pt4 had dropped ~58% to just ~1.23, of which ~82% desorbed intact, with the remainder decomposing to deposit C and liberate D2. For the Pt4/alumina sample, the total D2 desorption during the 6 TPD runs corresponded to the deposition of ~5.75 C atoms per deposited Pt4 cluster.