Straight-chain hydrocarbons and slightly branched chain hydrocarbons are selectively converted utilizing highly siliceous porous crystalline materials of the zeolite type having SiO.sub.2 /Al.sub.2 O.sub.3 ratio of greater than 200, unique molecular sieving properties and superior resistance to ammonia deactivation. The catalyst preferably contains acidic cations and can also contain a component having a hydrogenation/dehydrogenation function. The process of this invention is particularly useful for the dewaxing of hydrocarbon oils, including removal of high freezing point paraffins from jet fuel to lower freezing point, as well as improving the octane rating of naphtha fractions.