For the first time, the formation of paramagnetic defect centers in AlPO4 molecular sieves has been monitored by electron spin resonance (ESR) spectroscopy. Results from ESR measurements demonstrate that the structural defects in hydrated AlPO4-5 and VPI-5 molecular sieves change into paramagnetic defect centers when the molecular sieves are dehydroxylated under a high vacuum (similar to 10(-5) Torr) at 773 K. Adsorption of O-2 or NO on dehydroxylated molecular sieves causes an immediate loss of the ESR signals from paramagnetic defects. This suggests that the defect centers formed are F-type centers, which can serve as electron donors for electron-withdrawing adsorbates.