The effect of pretreatment (O-2 or H-2) and catalyst history was investigated through room temperature ethylene hydrogenation reaction over several types of platinum based nanoparticle systems: 1.6 nm Pt/TTAB, 4.1 nm Pt/PVP (with and without UV treatment), 4.1 nm Pt with a silica shell, and e-beam evaporated Pt thin films were tested. The H-2 pretreatment resulted in the absence of activity. However, Pt active sites for the ethylene hydrogenation reaction were recovered after an O-2 pretreatment irrespective of the catalyst history, regardless of the particle size nor the presence, absence or type of capping agent. The calculation of the average TOF resulted in 10.13 +/- A 3.27. This value correlates well with data from the literature. Thus, the ethylene hydrogenation reaction can be used to determine available sites of Pt catalysts if the reaction is following an O-2 pretreatment. .