Particle size and its distribution in powdered samples presents potential problems for accurate measurement in diffuse reflectance infrared Fourier transform spectrometry. Applications of the Kubelka-Munk theory to calculate the infrared spectrum from diffuse reflectance data assumes that the sample is a continuous medium. Particle size is not accounted for in the transformation. However, powdered samples of the same concentrations but with different particle sizes give different diffuse reflectance spectra. A qualitative interpretation of the effects of particle size on the diffuse reflectance spectra was made by measuring different concentrations of miztures of potassium bromide and polystyrene spheres with defined diameters. Principal component analysis was used to show the differences in the intensities of absorption with particle size variation. Spectral similarity (or dissimilarity) of the polystyrene spheres as a function of diameter was easy to compare using the score plots of the multivariate data. Additionally, the linearity of the Kubelka-Munk function was tested for three different particle sizes and concentrations. The Kubelka-Munk seems to be linear for lower concentrations. Deviation from Kubelka-Munk function starts at an early stage for the smaller particles.