Thermogravimetric adsorption data for CzH4 and H 2 0 and i.r. and n.m.r. spectroscopic results are combined to obtain information on the role played by acidic Bronsted sites in the H-ZSM-5 zeolite in the adsorption and activation of C2H4. Water and ethene compete for such sites in various ways which depend on the pretreatment of the zeolite. Physisorbed water prevents the adsorption of C2H4. Ethene is readily activated and reacts on the Bronsted acidic sites, even at room temperature. There is further evidence for the formation of carbenium-type intermediates.