This method covers the measurement of the oxidation signal in an asphalt mixture by a portable infrared spectrometer (PIRS) equipped with a diffuse reflectance accessory. Three oxidation signals are compared: (1) the in-place pavement, (2) asphalt mixture production sample, and (3) the approved job mixture formula specimen. The oxidation signals are determined from a specified peak value from the absorbance spectrum of the asphalt samples. The in-place pavement PIRS oxidation signal can be used to assess its aging rate. The pavement aging rate is determined by comparing its initial PIRS oxidation signal with successive PIRS oxidation signals obtained at some defined frequency.