Production of light olefins

The process for making light olefins containing 2 to 4 carbon atoms which comprise contacting a feedstock comprising one or more aliphatic hetero compounds with a silicoaluminophosphate molecular sieve in the presence of a diluent wherein the silicoaluminophosphate molecular sieve comprises a microporous crystalline silicoaluminophosphate whose unit empirical formula in the as-synthesized and anhydrous form is

• mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
• wherein "R" represents at least one organic templating agent present in the intracrystalline pore system;
• "m" has a value of from 0.02 to 0.3;
• "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z)O.sub.2 ;
• "x", "y" and "z" represent the mole fractions of silicon, aluminum and phosphorus respectively, present as tetrahedral units, said mole fractions being such that they are within the pentagonal compositional area defined by points A, B, C, D and E of the ternary diagram which is FIG. 1, at effective process conditions to produce said light olefins, comprising the improvement of correlating the diluent to the silicoaluminophosphate whereby the average kinetic diameter of the diluent molecules is greater than the average pore size of the silicoaluminophosphate molecular sieve.