Silicoaluminophosphate molecular sieves of SAPO-5 type are synthesized with variable Si/(Al + P + Si) mole ratios using triethylamine as the organic additive. The crystalline products are characterized by several physico-chemical methods. Incorporation of silicon into the framework of SAPO-5 is demonstrated by the variation in the unit-cell volume with silicon mole fraction, chemical analysis and magic-angle-spinning nuclear magnetic resonance. When Si/(Al + P + Si) < 0.077, silicon substitutes predominantly for phosphorus. For compositions where Si/(Al + P + Si) > 0.077, it appears that a combination of mechanisms, substitution of one silicon for one phosphorus and substitution of two silicons for aluminium–phosphorus pairs, takes place. Surface analysis using depth profiling shows an inhomogeneous distribution of silicon at higher silicon content. Preliminary catalytic results in o- and m-xylene isomerization are discussed.