Ethylene-D4, 14C-ethylene and tritium have been used as tracer molecules in a study of the nature and reactivity of adsorbed species on several different nickel catalysts. Reactions have been carried out on alumina- and silica-supported metal catalysts at nickel concentrations of 0.1 and 5%, a nickel powder catalyst, and a nickel catalyst partially poisoned by nickel phosphide in an attempt to determine the effect of the form of the metal on the catalyst reactivity. As well as the metal catalysts, the support materials themselves have formed part of the study. The catalysts were contained in a microcatalytic reactor which formed part of a flow system and the reaction gases were injected onto the catalysts after reduction.