Temperature-programmed desorption (TPD) and electron spin resonance (ESR) techniques have been employed to investigate the oxygen adsorbed on the paramagnetic defect centers in dehydroxylated AlPO4-20, AlPO4-11, AlPO4-5, and VPI-5 molecular sieves. Two different adsorption sites are observed for the small-pore AlPO4-20, where oxygen is entrapped within the intracrystalline voids generated during the calcination step, as well as within the beta-cages. However, the unidimensional molecular sieves AlPO4-11 AlPO4-5, and VPI-5 have only one type of adsorption site within their inner pores. The apparent activation energies of desorption (12.0-21.7 kcal mol(-1)) determined from the oxygen TPD results demonstrate that, as the pore size of the AlPO4 molecular sieve is larger, the interaction between the adsorbed oxygen and the molecular sieve framework becomes weaker.