The effects of moisture on the structure of two commercial (batch A and batch B) and one pure grade (batch C) of magnesium stearate have been examined by using differential scanning calorimetry (DSC), thermogravimetry (TG), X-ray powder diffractometry (XRD) and thermomicroscopy (HSM). The results suggest that the role of water is different in both of the commercial batches. Powder A showed a two-stage mass loss at temperatures between 25-degrees-C and 100-degrees-C indicating that the water present was tightly bound. The scans from batch B exhibited a continual mass decrease indicating that the moisture present was more loosely bound. With high purity magnesium stearate (batch C) water was bound in the ratio of two molecules to every one of the fatty acid salt. During storage to hydrate at 76% RH and 93% RH, powder A became more crystalline while powder B remained amorphous. Magnesium stearate dihydrate exhibited no tendency to form a trihydrate or higher hydrated forms. A short review of previous studies on magnesium stearate is given in the Introduction.