XRD-pure SAPO-31 and SAPO-11 molecular sieves were obtained from the same synthesis gel, free of seeding, with a relatively low DPA/P2O5 ratio (0.3) after 40 and 120 h of crystallization, respectively, at 353 K. The phase transformation sequence was closely followed by XRD. The initial formation of the dense phases berlinite (B) and cristobalite (C) in the starting gel at a defined proportion seems to play an important role in directing the synthesis toward the desired molecular sieves. Structural factors were invoked to explain differences in the selectivity toward isobutene, observed for samples of SAPO-31 and SAPO-11 with similar acidity characteristics, during the transformation of 1-butene at 743 K and WHSV of 1.8 h(-1). (C) Elsevier Science Inc. 1997.