A new method for the determination of aluminum and silicon has been developed for zeolite catalysts. In contrast to previous methods, thermal neutrons are used for the analysis of both elements, and cadmium absorbers are not needed. The silicon determination utilizes a one-hour irradiation to observe the31Si produced by the (n, ) reaction of30Si. A 15-second irradiation is used for the27Al(n, )28Al reaction. The28Al activity is corrected for the contribution from the28Si(n,p)28Al reaction by using the analyzed weight of silicon in the sample and the data for a silicon standard irradiated simultaneously with the zeolite and the aluminum standard. The quantitation limits are 0.012 g for silicon and 3.3×10–5 g for aluminum. Sodium presents a significant interference, but this element can be removed by taking advantage of the ion exchange properties of these materials.