Abstract: 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 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 incorporate amorphous silica therein.