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A quantitative measurement method for the determination of fine-particle bisulfate in ammonium bisulfate collected from the ambient air onto Teflon® filters is described. Infrared absorbance measurements of the Teflon® filters are made before and after particle collection. Subtraction of the two spectra reveals the absorbance spectrum of the particles. The presence of bisulfate is identified by characteristic and unique spectral features including prominent absorption bands at 1050 and 870 cm−1. The integrated band absorbance at 870 cm−1 is used for quantitation. Wet chemical measurements of sulfate and hydrogen ion in synthetically generated ammonium bisulfate standards are used in calibration. The lower limit of detection for the bisulfate ion is 150 nanomoles. This amount corresponds to the ammonium bisulfate which would be collected from an air volume containing 1.2-microgram/m3 samples for 24 hours at 10.0 L/min. This method provides a specific, nondestructive, direct measurement of ammonium bisulfate. As such, the method has distinct advantages over indirect methods for the determination of bisulfate ion.