Thermal and physical properties of vegetable grade magnesium stearate monohydrate, dihydrate and trihydrate have been studied. The samples have been studied as received, after moisture treatment (3 and 6 weeks at 96%RH) and after degassing (2 h in vacuum at 40 and 105 degrees C). The specific surface area, average pore size, thermal properties and crystal structure were dependent on the hydration state of the samples. All of the studied properties changed crucially after the samples were degassed at 105 degreesC. The biggest changes were in magnesium stearate trihydrate whose specific surface area after degassing at 105 degreesC was only a fifth of the area of the as-received sample. The results of the thermal analysis and X-ray diffraction measurements show that all of the studied magnesium stearates changed to the anhydrate after degassing in vacuum at 105 degreesC. A gravimetric method was used to examine the water sorption behavior of the samples and results show that part of the water absorbed at room temperature remains in the structure of magnesium stearate. No changes in the hydration states were observed, whereas properties were different compared to the as-received samples. The monohydrate sample changed most in the moisture treatment. All of the measured nitrogen adsorption-desorption isotherms except the isotherms measured from the anhydrated samples did not fit properly to any known theory, which might be a result of interaction of nitrogen and water. (C) 2004 Elsevier B.V. All rights reserved.