The catalysis of cyclopentane–deuterium exchange by nickel–copper alloy films persists to low (4 atom %) nickel content. Activation energies where measurable and product distribution patterns are constant for nickel contents greater than ca. 24 atom % nickel, but below this value there is a marked increase in over-all activation energy and an even-numbered distribution pattern (maxima for tetra- and hexa-deuterio-products) develops. The latter feature is ascribed to doubly bonded intermediate described as ααββ- and ααββγγ-adsorbed. These are the species believed to be responsible for the severe self-poisoning found on pure nickel. The strength of their adsorption decreases with the increasing fraction of copper, causing initially a decrease in the extent of self-poisoning and ultimately the appearance of the even-numbered deuteriocyclopentane exchange products. These features are discussed in terms of compositional and texture changes.