Two small-pore silicon-substituted molecular sieves SAPO-34 and SAPO-44 with varying amount of silicon have been synthesized and characterized by XRD, SEM, FT-IR spectroscopy, and thermal analyses. Their acidity has been measured quantitatively by temperature-programmed desorption of ammonia. The results of these studies along with solid-state MAS NMR results show that the major part of silicon substitutes isolated phosphorus atoms in the structure whereas some amount of it forms small silica patches. The extent of formation of such small patches is more noticeable in SAPO-34 than in SAPO-44.