Acidity of the catalysts plays a crucial role in the skeletal isomerisation of n-butene. Very weakly acidic OH groups are not active, very strongly acidic groups are active but they induce a too high extent of the undesired side reactions. This paper addresses the question to which extent and in which way the selectivity can be manipulated by changes on the catalyst. One can either vary the OH acid strength, by using different three-valent ions in the skeleton of, e.g., ZSM-5, or to change the total number of OH groups, by variation in the Si/Al (Si/M)-ratio. Both ways are explored here and it is concluded that when a selectivity change is achieved, the dominating effect is that of the density of the active sites. This is not a primary mechanistic effect but an effect of variations in the apparent contact time on the extent of side reactions.