A modified zeolite catalyst derived from a zeolite of a structural type which consists of a one-dimensional micropore structure of channels made from rings containing between 8 and 12 silicon/aluminum atoms is disclosed. It consists substantially of a plurality of crystallites having additional mesoporosity whose volume is in the range 0.09 to 0.25 cc/g as mentioned by nitrogen adsorption at 77° K and calculated by the BJH method. The mesoporosity may be introduced into the crystallites by e.g. treatment with aqueous sodium hydroxide at a pH at 25° C. in excess of 8 for an extended period at elevated temperature. The catalyst shows improved resistance to catalyst deactivation and greater selectivity to higher hydrocarbons when used to e.g. oligomerize light alkenes e.g. propene or the butenes.