[en] Isotopic exchange between cyclopentane and deuterium, mostly at 81⁰C, has been investigated on a set of 10 Pt/SiO₂ catalysts whose percentages exposed ranged from 6.3 to 81%. The standard pretreatment, O₂, 300⁰C; H₂, 300⁰C; He 450⁰C with cooling in helium, leads to turnover numbers for formation of exchanged cyclopentanes, which decline by a factor of less than 2 as the percentage exposed increases from 6.3 to 63.5% and then at 81% rise to about the value for 6.3%. Selectivity to various exchanged species varies with percentage exposed and, in particular, D₆/D₁₀ (the ratio of initial yields of d₆ and d₁₀) decreases substantially as the percentage exposed increases. This suggests that exchange of the hydrogen atoms on both sides of the cyclopentane ring proceeds mainly via formation of α,α-diadsorbed cyclopentane on catalysts with a smaller percentage exposed and by rollover on catalysts with a large percentage exposed. Other pretreatment conditions such as O₂, 300⁰C; H₂, 100⁰C and O₂, 300⁰C; H₂, 450⁰C with cooling in hydrogen give larger structure sensitivites for activity and different selectivities. Catalysts deactivate rather rapidly but with little change in selectivity. Deactivation is accompanied by the formation of carbonaceous residues. C/Pt/sub s/ becomes approximately 0.5 and the activity declines 10% in about 10 min on stream. Ninety percent or more of the carbon in the residue occurs in a cyclopentane structure because 90% of the carbon is removed as cyclopentane by hydrogen at 100⁰C. Repetition of the standard pretreatment regenerates the initial activity and selectivity