The F 1s spectra from fluorine-containing phosphate glasses provide quantitative information about the nature of fluorine bonding that can be used to test structure-composition models. when fluorine is added to an aluminophosphate base glass, Al-F bonds are initially preferred until all available Al sites are filled, after which additional F replaces P-O-P bonds to form P-F sites. in tin phosphate base glasses, the P-O-P bonds are consumed during the initial stages of fluoridation, followed then by the formation of Sn-F bonds. the structural insight provided by XPS is complementary to that provided by more conventional “bulk” spectroscopic probes.