Process Analytical Technologies (PAT) recommended by the Food and Drug Administration (FDA) have significantly increased during the past years in the design, control and monitoring of pharmaceutical manufacturing processes [1]. Nowadays PAT is also commonly used in Good Manufacturing Practices (GMP). Some PAT techniques employ on-line fiber-optic sensors to acquire non-destructive measurements of physical properties, kinetic information about dissolved molecule and solid fraction of particles in slurries, in real time [2]. <br><br> This study uses attenuated total reflectance ultra-violet visible (ATR UV-vis) spectroscopy for kinetic modeling of the dissolution of salicylic acid in ethanol-water within a small-scale semi-batch reactor. The dissolution model, which relies on a power-law equation, can be obtained by adding aliquots of an ethanol-water mixture into a salicylic acid slurry. <br><br> Near-infrared (NIR) diffuse reflectance spectroscopy is used to detect the degree of saturation and quantify the solid fraction using a classical Kubelka-Munk transformation followed by a calibration. A temperature probe is also used to monitor heat changes involved in this dissolution. <br><br> [1] Fevotte G., International Journal of Pharmaceutics 241 (2002) 263-278 <br> [2] Gemperline et al, Analytical Chemistry 76 (2004) 2575-2582