To selectively isomerize an olefin compound, esp. n-butene, by using silicalite subjected to specific pretreatment as a catalyst.

(A) A high-purity steam or steam diluted with an inert gas (e.g. nitrogen) is passed on (B) silicalite at 300-750 (pref. 450-600)°C, followed by feeding a (C) feedstock containing (i) at least 10vol.% of n-butene or further containing (ii) isobutene in the vol. ratio (ii)/(i) of (1:2) (pref. <1:2, optimally <1:10) to pass the component C on the component B at ≥500 (pref. 500-650)°C, thus recovering a stream containing an increased amount of isomerized component (ii).