The interpretation of near infrared (NIR) reflectance in terms of material composition is complicated by particle size variation. An investigation is reported into the relationship between NIR reflectance and particle size using data from ground wheat samples. It is shown that the sensitivity of reflectance to particle size is dependent on the absorption properties of the material at the sampling wavelength. A theory relating a function of reflectance, known as the Kubelka–Munk function, to particle size suggests that the effects due to changing wheat composition can be isolated from the particle size effects using normalized Kubelka–Munk functions. Analysis of the spectral data from the ground wheat samples supports this theory. The ratio of Kubelka–Munk functions at appropriate absorption and reference wavelengths is subsequently manipulated to yield single term calibration equations for the protein and water in the wheat samples. Comments are made on the implication of the reported findings for various calibration techniques.