A catalyst is provided, which is especially applicable for the selective production of C.sub.2 -C.sub.3 olefins, comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index, as hereinafter defined, within the approximate range of 1 to 12 and having contained within the interior crystalline structure thereof added amorphous silica in an amount of at least about 0.3 and preferably between about 0.5 and about 30 weight percent. The resultant catalyst is characterized by a n-hexane sorption capacity at a temperature of 90.degree. C. and a n-hexane partial pressure of 83 mm. of mercury which is at least 1 percent less than corresponding sorption capacity under identical conditions for the unmodified zeolite. Generally, the n-hexane sorption capacity for the described silica-modified zeolite, at the above specified conditions, is 5 to 60 percent less than that for the zeolite which has not undergone treatment to incorpoate amorphous silica therein. The invention described herein also encompasses synthesis of the specified catalyst and use of the same in selectively converting lower monohydric alcohols and their ethers, especially methanol and dimethyl ether, to a hydrocarbon mixture rich in ethylene and propylene.