The surface properties of supported tungsten oxide catalysts, pure tungsten oxide, and pure alumina were studied by IR spectroscopy, using pyridine and carbon monoxide as probes. The interaction of these catalysts with n-butenes, molecules which are isomerised selectively by the catalysts to isobutene, has also been studied by IR spectroscopy. A correlation is found between the catalytic properties of the catalysts and the strength and nature of the acidic sites found on their surface.