The effect of type and concentration of modifiers on the physicochemical and functional properties of magnesium hydroxide obtained by precipitation, and its calcinate magnesium oxide, was studied. Magnesium hydroxide obtained from magnesium sulfate and sodium hydroxide was subjected to thermal decomposition at 450 degrees C. The precipitation reaction took place in optimised conditions with the use of a modifier at suitably chosen concentration and molecular mass. The modifiers were non-ionic compounds representing poly(ethylene glycols): PEG 200, 8000, 20000. Detailed and comprehensive characterisation of the unmodified and modified samples, either hydrated or calcined was made. The magnesium hydroxide samples obtained in this study show plate morphology of particles. The modifier introduced into the reaction system had a direct influence on the morphological and dispersive properties of the products as well as their surface character and specific surface area. However, it was proved that the presence of a modifier did not change the crystalline structure of the products. A mechanism of PEG activity explaining the effect of different concentrations of the modifier on agglomerate formation was proposed. (C) 2012 Elsevier B.V. All rights reserved.