The solvent mediated anhydrous to monohydrate phase transition of citric acid was monitored using in situ Raman spectroscopy and image analysis. The solid phase composition, solute concentration and CSD (Crystal Size Distribution) were measured during specific experiments designed to study the process: dissolution of the metastable anhydrous form and nucleation and growth of the monohydrate stable form. A Population Balance Model was developed to depict the time evolutions of the populations of particles involved. Nucleation and growth kinetic parameters were estimated through the non-linear least squares minimization of model-experiments prediction errors. The overall model is shown to satisfactorily describe the solvent mediated phase transitions process. Secondary nucleation of stable monohydrate particles was found to depend on both the supersaturation and the overall solid content. As far as the nucleation of stable particles in the presence of metastable solid is concerned, the rate of particle generation exhibits identical dependency on the metastable and stable solid content; a feature which remains to be further investigated and explained.