The focus of this study was the understanding of the hydrate transformations of anhydrous olanzapine Forms I and II (the most common polymorphs) upon exposure to different moisture conditions (11, 53, 75, 93% RH) and direct contact with water (e.g. aqueous slurry) and the impact of hydration on the aqueous dissolution rates of the polymorphs. The kinetics of reversible transformations (anhydrate-hydrate phases) and the identification of polymorphs were evaluated by differential scanning calorimetry, thermogravimetry, infrared (DRIFT) and X-ray powder diffraction. The results showed that anhydrous Forms I and II have undergone water vapor phase induced transformations at 93% and 75% RH, respectively. At 93% RH Forms I and II showed to hydrate into dihydrates D and B, respectively, the latter with a higher hydration rate. The conversion of Form I into the dihydrate D showed to affect the dissolution rate of olanzapine (f2 < 50). As slurries both forms showed to hydrate into a mixture of two different Forms – dihydrate B and higher hydrate. The study provided an understanding of the conversion pathways of the different forms when they were exposed to humid air or aqueous environments, resembling the transformations that might occur during processing, storage or during the persecution of dissolution tests to assess the quality of dosage forms delivering olanzapine.