A process for the double bond isomerization of a vinylidene olefin, comprising contacting a feed comprising the vinylidene olefin with an isomerization catalyst which comprises a molecular sieve in an acidic form, which molecular sieve comprises pores which have a pore size of more than 0.6 nm; and a process for treating an olefin mixture which comprises a linear .alpha.-olefin and a vinylidene olefin which is isomeric to the linear .alpha.-olefin and which is of the general formula CH.sub.2.dbd.C(R.sup.1)R.sup.2, wherein R.sup.1 represents an ethyl group and R.sup.2 represents a linear 1-alkyl group, which process comprises isomerizing the vinylidene olefin to form a double bond isomer of the vinylidene olefin by contacting a feed comprising the olefin mixture with an isomerization catalyst which comprises a molecular sieve in an acidic form, which molecular sieve comprises pores which have a pore size of more than 0.6 nm, and separating the linear .alpha.-olefin from the double bond isomer of the vinylidene olefin.