Fourier transform infrared spectroscopy has been employed to investigate the effect of both the deposition parameters and subsequent thermal processing on phosphosilicate glass (PSG) and how these modifications are reflected in the peaks corresponding to the Si-O, P-O, and P=O bonds. The variation of the frequency shift of the Si-O main band with the phosphorus content and the annealing of PSG has been clearly established. Furthermore, it is suggested that there is no separate phosphorus pentaoxide or trioxide present in the glass and that both P=O and P-O bonds determine the properties of PSG. It has been determined that the high etch rate of as-deposited PSG is due mainly to its porosity, its distorted and disordered internal structure, and the initially hydrolized P=O bonds. The decreased etch rate after annealing is due to the decrease in porosity the restoration of the P=O bonds, and probably the formation of insoluble compounds. Annealing at 950 or 1000 degrees C yields significant changes in the structure of the PSG and a considerable phosphorus out-diffusion. In contrast, annealing at 850 degrees C is sufficient to stabilize the structure of the PSG without important structural modifications.