Silicoaluminophosphate molecular sieves of SAPO-11 type were synthesized with variation in the [Si/(Al + Si + P)] ratio, using the hydrothermal method, starting from silica, pseudobohemite, orthophosphoric acid and water, in the presence of a di-isopropylamine organic template. The samples were characterized by elemental analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM). The incorporation of silicon into the framework of SAPO-11 is demonstrated by the variation of the unit-cell volume with the silicon content. CELREF software was used to index and refine the main lines of the XRD patterns. It is shown that the unit-cell volume decreases over a wide range of silicon content.