Commercial grades of magnesium stearate have been analysed by nitrogen adsorption having been pre-treated at temperatures between 30 °C and 110 °C and in the as-received state. Characteristics of nitrogen adsorption/desorption isotherms are assessed through the linearity of low relative pressure isotherm data and the BET transform plot together with the extent of isotherm hysteresis. Comparison is made between thermal gravimetric analysis and mass loss on drying. Features of gas adsorption isotherms considered atypical are identified and possible causes presented. It is shown that atypical isotherm features and issues of applying BET theory to the calculation of SBET are linked to the presence of hydrated water and that these depend on the hydration state: being more pronounced for the di-hydrate than the mono-hydrate. Dehydration reduces the extent of atypical features. SBET of a mono-hydrate sample is 5.6 m2 g−1 and 3.2 m2 g−1 at 40 °C and 100 °C degassing respectively but 23.9 m2 g1 and 5.9 m2 g−1 for di-hydrate containing samples under comparable degassing. Di-hydrated samples also show SBET >15 m2g1, BET C-values <7 and BET correlation coefficients <0.98 before dehydration. Possible mechanisms for atypical isotherms are critically discussed together with the suitability of applying BET theory to nitrogen adsorption data.