Results of ir investigations are reported on some surface properties of a fumed silica (Cab-O-Sil) support, a Pt catalyst made by impregnation (Pt/CS-I) and two Cu catalysts made by precipitation (Cu/CS-P) or ion-exchange (Cu/CS-X). The support and the catalysts were studied in the OHOD exchange reaction of surface OH groups by consecutive treatment with H 2 and D 2 or H 2 and D 2O. The behaviour of the Cu and Pt catalysts was investigated in the isomerization of methylenecyclohexane. The effects of 1,1-dimethylcyclopropane on the catalysts and the support in the presence or absence of D 2 were also examined. The major findings were as follows: (i) OHOD exchange with D 2 is negligible for the support at room temperature, but the reaction is practically complete at 673 K, (ii) exchange is fast over Pt/CS-I, even at room temperature, but both Cu/CS catalysts are considerably less active, (iii) at ambient temperature, exchange with D 2O proceeds almost to completion over the support and over each catalyst, (iv) isomerization of methylenecyclohexane occurs over Pt/CS-I, but not over the copper catalysts, (v) 1,1-dimethylcyclopropane adsorbs dissociatively over each catalyst without D 2, (vi) in the presence of D 2, however, it reacts through associatively adsorbed species over the copper catalysts, while a dissociative mechanism is observed together with the predominant associative ring opening over Pt/CS-I.