Adsorption and reaction of 1-butene, cis- and trans-2-butene and isobutene on H-ZSM-5 and H-FER were studied by FF-IR spectroscopy between 30-450 degreesC. n-Butenes were monomolecularly adsorbed via pi -bonding on H-FER at 30 degreesC. At elevated temperatures butadiene was monitored on H-FER, which was then transformed into dialkyldiphenyl. On H-ZSM-5 all butene isomers were adsorbed via pi -bonds, followed by dimerization. From these results and from catalytic measurements a monomolecular pathway is proposed on H-FER and a bimolecular one on H-ZSM-5.