Two spatially distinct types of UV-induced refractive index changes in phosphosilicate glass fibers have been differentiated by the authors. The index change close to the core-cladding interface is less stable than the index change in the bulk fiber core. It is shown that by using presensitization techniques, in which the use of hydrogen is minimized so as to optimize the index changes possible within the core center, gratings of better stability than those prepared in fully hydrogen-loaded optical fiber are obtained.