Fourier transform infrared spectroscopy has been used to study quantitatively the acetylation of monodisperse polystyrene microspheres with diameters ranging from 7 to 9μm. The CH2stretching infrared vibration mode at 2921 cm−1was used as the internal intensity standard. The acetylation extent could be easily measured by comparing the relative intensities of the bands of acetyl group (1678, 1415, 1359 cm−1) or the bands due to thepara-substituted benzene ring to the band at 2921 cm−1from the calibration curve.