Abstract: Diffuse reflectance infrared (IR) spectra show that pressing kaolinite into pellets for transmission IR studies changes relative intensities of hydroxyl peaks between 3700–3600 cm−1. The magnitude of change depends on the absolute pressure, the pressing time and whether a salt matrix is used. Pressing with KBr causes larger differences than pressing neat. Diffuse reflectance IR does not require sample pressing, eliminating this variable in kaolinite studies. This improves IR's ability to distinguish kaolinites according to their hydroxyl group differences and may lead to a better understanding of kaolinite hydroxyl structures.