When suspended in water, anhydrous carbamazepine (C15H12N2O; 1) was transformed to carbamazepine dihydrate (C15H12N2O · 2H2O; 2). Compound 1 was dispersed in water at 25 °C, and an aliquot of the slurry was taken at periodic intervals and immediately filtered under conditions which removed all the physically bound water from the solid. The weight fractions of 1 and 2 in the solid were quantified by two methods: (a) by determining the weight loss on drying the solid at 60 °C under reduced pressure to a constant weight and (b) by a quantitative powder X‐ray diffraction technique. There was good agreement between the above two methods. More than 95% (w/w) of 1 was converted to 2 in 1 h and the decrease in the weight fraction of 1 with time was approximately a first‐order process.