Borophosphosilicate glass (BPSG) dielectric thin films deposited on both bare and oxide-coated undoped silicon wafers have been analyzed using infrared external reflection-absorption spectroscopy (IRRAS). The partial least-squares (PLS1) algorithm was used to simultaneously determine boron content, phosphorous content, and film thickness, with standard errors of prediction of 0.08 wt %, 0.11 wt %, and 24 Angstrom, respectively, in the BPSG films on oxide-coated wafers (similar results were obtained with the bare wafer BPSG sample set). These results were statistically equivalent to the precisions of the reference methods used to determine each BPSG property, indicating that the precisions of the PLS1 models were limited by the precisions of the reference methods. IRRAS reproducibility and repeatability results verified that the method can be more precise than the reference methods. The reproducibility results were derived from the standard deviation of ten PLS1 predictions of ten IRRAS spectra that were obtained from a single BPSG sample that was moved in and out of the sample chamber between each spectral measurement. The repeatability results were obtained similarly, but the sample was not moved between acquiring the ten spectra. The precision of the IRRAS method from the repeatability data was found to be +/-0.006 wt % B, +/-0.011 wt % P, and +/-4 Angstrom film thickness. The reproducibility results were generally less precise than the repeatability results. Studies done as a function of spectral resolution and signal averaging showed that very rapid IRRAS measurements could be made (up to 2 Hz) with high PLS1 prediction precision for the three calibrated BPSG properties. The results show that the IRRAS technique has great potential for rapid, at-line quality control monitoring of BPSG thin films on undoped silicon wafers.