Silicoaluminophosphate (SAPO)-34 molecular sieves modified with transition metals Co, Mn, and Ni were investigated for their activity, selectivity, and lifetime in the methanol to olefins (MTO) reaction and compared with that of unmodified SAPO-34. The electronic state and location of the metal ions in these materials were determined by magnetic measurements (temperature and magnetic field variations of magnetization). These magnetic studies show that for MnSAPO-34 and CoSAPO-34 metal ions are incorporated into the SAPO framework with concentrations similar or equal to 0.12 and 0.17 wt.%, respectively. Nickel modified SAPO-34 samples were prepared by two different procedures. For NiSAPO-34 prepared by one procedure (Method I), the magnetic studies show the presence of nanoparticles (Ni or NiO) outside the SAPO framework. For NiSAPO-34 prepared by a second procedure (Method II), these studies show that the nickel ions are incorporated into the SAPO framework. Although the total C-2-C-4 olefins activity on these materials was comparable to that of unmodified SAPO-34, significant variations in the deactivation behavior were observed among various metal-modified catalysts. MnSAPO-34 was found to be the best catalyst based on catalyst lifetime. In the case of Ni-SAPO-34 (Method 1), unusually high amounts of methane were observed in the products due to the presence of nanoparticles outside the SAPO framework. (C) 2003 Elsevier Science B.V. All rights reserved.