In-line monitoring of solid-state properties in crystallization processes is of great significance in controlling the quality of crystalline active pharmaceutical ingredients. In this work, the solvent-mediated phase transformation of anhydrous to dihydrated carbamazepine in ethanol-water mixtures was studied using an in-line Raman immersion probe. The solute concentration profile was measured by off-line sampling. The transformation experiments were conducted with different operation parameters in terms of solvent composition and temperature. The transformation rate depends on both solvent composition and temperature. The mechanism of the transition was interpreted with the two-step polymorphic form transformation mechanism. It was observed that the crystallization of the stable form was the rate-controlling step. The influence of the operation parameters on the transformation rate can be interpreted as the effects of solvent and supersaturation on the crystallization kinetics. Another interpretation is proposed by correlating the deviation of the water activity from the equilibrium value to the rate of phase transformation. It was observed that the correlation of the water activity deviation and the phase transformation rate was independent of solvent composition and temperature.