The physicochemical and thermal properties of various forms of magnesium stearate were examined to establish their relationship with its lubricant abilities. The form of magnesium stearate having poor lubricating properties in commercial samples could be differentiated using differential scanning calorimetry (DSC). The lubricating properties were decreased in milled, dried and stored samples. These properties depend on the moisture content and the total enthalpy as seen from the DSC peaks corresponding to desorption of water, with the latter having a greater effect. The mechanism of lubrication seemed to involve water and/or gas molecules entering the spaces of the crystal lattice, causing a decrease in the interactive forces of the crystal lattice which, in turn, leads to easier shearing of the lubricant powder particles.