Near-infrared spectroscopy was used to monitor the phase conversion for two solvatomorphs of caffeine, an anhydrous form and a nonstoichiometric hydrate, as a function of time, temperature, and relative humidity. The transformation kinetics between these caffeine forms was determined to increase with temperature. The rate of conversion was also determined to be dependent on the difference between the observed relative humidity and the equilibrium water activity of the anhydrate/hydrate system, that is, phase boundary. Near the phase boundary, minimal conversion between the anhydrous and hydrated forms of caffeine was detected. Using this kinetic data, the phase boundary for these forms was determined to be approximately 67% RH at 108C, 74.5% RH at 258C, and 86% RH at 408C. At each specified temperature, anhydrous caffeine is the thermodynamically stable form below this relative humidity and the hydrate is stable above. The phase boundary data were then fitted using a second order polynomial to determine the stability relationship between anhydrous caffeine and its hydrate at additional temperatures. This approach can be used to rapidly determine the stability relationship for solvatomorphs as well as the relative kinetics of their interconversion. Both of these factors are critical in selecting the development form, designing appropriate stability studies, and developing robust conditions for the preparation and packaging of the API and formulated drug product.