The objectives were (1). to perform solid-state characterization of anhydrous betaine (A) and betaine monohydrate (M), (2). to develop a pressure differential scanning calorimetric (DSC) technique for the quantification of M when present as a minor component in a mixture with A and, (3). to study the effect of annealing of A on the kinetics of A --> M transition. X-ray powder diffractometer (XRD), DSC, thermogravimetric analyzer (TGA), and an automated moisture sorption apparatus were used to characterize the phases. DSC at an elevated pressure of 200 psi enabled quantification of M in mixtures of A and M. Humidity-controlled TGA allowed study of the kinetics of A --> M transition. Automated moisture studies showed that A has a strong tendency to sorb water (at RH >or= 20%, 25 degrees C) and convert to M. When M was subjected to DSC at ambient pressure, the endotherms due to dehydration and vaporization of water overlapped. Pressure DSC enabled separation of these two thermal events. In mixtures of A and M, the enthalpy of dehydration (deltaH(d)) of M could be used for its quantification. A linear relationship was obtained when deltaH(d) was plotted as a function of the weight fraction of M in the mixture. The limits of detection and quantification of M in A were 0.15% and 1.5% w/w, respectively. The kinetics of water uptake by the annealed as well as the unannealed A, could be best described by the Avrami-Erofeev model (three-dimensional nucleation and growth). The calculated rate constant (k) of unannealed A (0.075 +/- 0.002 min(-1)) was significantly higher than that of annealed A (0.052 +/- 0.004 min(-1)). DSC at elevated pressure was a sensitive technique for quantification of M when present as a mixture with A. Annealing of A decelerated the A --> M phase transition reaction, possibly by increasing the degree of crystallinity of A.