Stearic acid coated samples of porous and non-porous grades of magnesium hydroxide and calcium carbonate have been characterized using diffuse reflectance infra red (DRIFT), and X-ray photoelectron spectroscopy (XPS). Using DRIFT, the signal due to the reacted coating reached a plateau at a coating level dependent on the filler surface area. XPS showed that coating thickness increased with the amount of coating, slope changes being observed in plots at lower levels of applied coating than those observed by DRIFT. The differences between the measurements for the two techniques is attributed to their different sampling depths. The FTIR having a greater sampling depth probes the internal surface area of the agglomerates present in the porous material, whereas XPS does not. The surface chemistry of the two types of coated filler is compared.