Fourier-transform infrared (FTIR) spectroscopy is a useful and nondestructive method for measuring the current atmospheric concentrations of inorganic compounds (sulfate, nitrate, and ammonium). It has been extensively used for environmental monitoring since the 1980s. In this study, we used FTIR spectroscopy to measure the inorganic compounds in particulate matter with a diameter of less than 2.5 µm and combined the data of gaseous pollutants (NO2 and SO2) to analyze the inorganic compounds in PM2.5 from January 24 to January 31, 2014, in Zhengzhou. The measurement period was divided into three pollution process stages. Stage 1 (January 24 to January 26) was the low-pollution stage, in which the wind from the east of Zhengzhou caused the pollutants to rapidly disperse and the haze to clear. During Stage 2 (January 26 to the noon of January 30), the sulfur oxidation ratio corrected poorly with SO42− (R2 = 0.45), suggesting that transportation was the main reason for the high sulfate concentration. The nitrogen oxidation ratio corrected well with nitrate (R2 = 0.91), suggesting that nitrate was formed through homogeneous gas–phase reactions of NO2 with OH or O3 to form HNO3 in PM2.5. During Stage 3 (afternoon of January 30 to January 31), the average concentration of PM2.5 changed from approximately 140 µg m–3 to 260 µg m–3, and the concentrations of sulfate, nitrate, and ammonium changed from 37.62 µg m–3, 56.63 µg m–3, and 34.63 µg m–3 to 32.14 µg m–3, 31.14 µg m–3, and 26.35 µg m–3, respectively. The hygroscopic growth of particles may be the primary reason for the high levels of PM2.5 during this period.