Reaction of filler grade magnesium hydroxide (Martinswerke Magnifin H10) with oleic acid (cis 9-octadecenoic acid) and stearic acid (octadecanoic acid) has been investigated using flow micro-calorimetry (FMC) and solution adsorption isotherm methods (both from heptane). Diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), sedimentation volume measurements and X-ray diffraction were also carried out on the isolated and dried adsorption isotherm treated samples. Stearic acid adsorbed to a greater extent (24 mg g(-1)) than oleic acid (12 mg g(-1)) and this is likely to correspond to vertical adsorption of both acids based on measured filler surface area and adsorbate molecular area. It is considered that stearic acid forms a layer more than one molecule in thickness and was shown by X-ray diffraction, in the dry state at least, to exhibit structural order. Oleic acid adsorbs as a single molecular layer with any additional (i.e. excess) acid present probably reacting fully with the magnesium ion, thus forming the double salt which then di;solves in the heptane. Magnesium stearate is not soluble in heptane. FTIR analysis of the supernatant liquors for the oleic acid treated filler showed no free acid, only magnesium oleate.