Several gamma-Al2O3-supported tungsten oxide catalysts have been studied in the skeletal isomerisation of l-butene. It appears that the presence of small tungsten oxide particles induces skeletal isomerisation. Large particles can be activated by prereduction treatment with hydrogen. It was shown that the selectivity and stability of the catalysts depend strongly on the tungstate loading. A sudden increase in selectivity is observed when dimeric WO42- species are formed. A low density of active sites presents deactivation, which is caused by carbonization of the catalyst due to oligomerisation of isobutene. The main by-products, C-3 and C-5 hydrocarbons, arise from the dimerisation and cracking of isobutene.