Nicotinic acid is crystallized by adding hydrochloric acid (HCl) to a sodium nicotinate aqueous solution. An attenuated total reflection Fourier transform infrared spectrometer is used to monitor the crystallization process of nicotinic acid. The partial least squares method is employed for IR data analysis. The solubility of nicotinic acid varies with the pH and the concentration of sodium nicotinate. The ability to measure the concentration and solubility of nicotinic acid in situ in aqueous mixtures of nicotinic acid and sodium nicotinate provides the possibility of controlling the degree of supersaturation of the solution and in turn optimizing the crystallization conditions for nicotinic acid.