The evolution of silicon processing technologies towards greater reliance on internal gettering by oxygen precipitates has led to the need for greater reproducibility in the measurement of the interstitial oxygen content of silicon slices. This measurement is presently being carried out with the use of either Fourier-transform or dispersive infrared spectrophotometers. This paper concerns the investigation of the effects of changing the apodization function and beam geometry on the quantitative determination of oxygen in silicon by Fourier-transform spectrophotometers. The apodization functions used include the boxcar, cosine, Happ-Genzel, and triangular functions. The beam geometry is varied by placing apertures between the interferometer and the silicon specimen. The effects of beam polarization and detector nonlinearity were also investigated.