Transformation of kaolinite to dickite is a common diagenetic reaction. The present report is part of a wider study to investigate the pathways of this polytype change. Fourier-transform infrared spectroscopy (FTIR) was used to attempt quantification of the relative proportions of kaolinite and dickite, validated by X-ray diffraction (XRD) results, in order to link mineral and structural features during the mineralogical changes. A group of kaolinite and dickite samples was investigated: 13 samples from the Frøy and Rind oil fields (North Sea), three kaolinite specimens with different crystal order and particle size (KGa-2, kaolinite API 17, Keokuk kaolinite), and two dickite-rich samples (Natural History Museum collection). Six FTIR spectral features were analyzed: (1) intensity ratio of the minima at 3675 and 3635 cm−1; (2) position of the band at ~1115 cm−1; (3) difference between the frequency of the bands at ~1030 and ~1000 cm−1; (4) intensity ratio of the bands generating shoulders at ~922 and ~900 cm−1; (5) position of the band at ~370 cm−1; and (6) intensity of the band at ~268 cm−1. Correlation of the features above with polytype relative proportions derived from XRD showed non-linear behavior, with maximum curvature at the dickite end, which precludes kaolinite-dickite quantification. Increasing kaolin particle size is known to cause decreased intensity of the FTIR spectra. A model was developed to test whether this effect is consistent with the non-linear progression of the IR features. The relative intensity of kaolinite and dickite IR features were calculated in a series of kaolinite-to-dickite transformations, where the size of particles increases with dickite proportion, and where dickite-dominated particles reach a larger size than kaolinite-dominated particles. The results indicated that the differential particle size increase is possibly the cause of the lack of linearity between IR- and XRD-measured dickite proportions.