Catalysts containing phosphorus, oxygen, silicon and - eventually - also aluminium, can be active in skeletal isomerisation of n-butene. The condition of activity is the presence of OH groups of the moderate acidity. The presence of strong Bronsted sites is undesired since these are expected to be responsible for by-product formation. No role of Lewis sites was observed with these catalysts. Low stability of ’open surface’ catalysts is caused by poisoning of active sites by carbonaceous layers. Deactivation can be suppressed by placing the active site in a confined space and a zeolitic channel with 10-membered rings fulfils such a condition.