A quantitative kinetics analysis of the solution-mediated phase transformations in aqueous-ethanol between the enantiotropically related alpha and beta polymorphs of p-aminobenzoic acid (PABA) is presented. A new temperature-controlled transmission flow-cell design for use with a laboratory X-ray source which can be utilized for diffraction or scattering experiments is described. This transmission cell in combination with in situ slurry X-ray diffraction is utilized to provide a quantitative analysis of the percentage by mass of respective solid phases in flowing suspensions, coupled with solution concentration as measured with UV–vis spectroscopy. The separate phases are identified through individual fingerprint peaks, the (1 0 3) reflection at 2θ = 15.6° relating to the alpha phase and the (0 2 2) reflection at 2θ = 25.3° for the beta phase. Examination of the conversion of a slurry of beta PABA in a 70:30 water/ethanol solvent mixture to the alpha polymorph, at a temperature range of 24–30 °C reveals t...