Supersaturation as the driving force of crystallization processes is an essential parameter and its effects on the quality of the product should be carefully investigated. In this paper, a systematic study on cooling rate and cooling mode effects on the supersaturation level and, consequently, on the size and shape of the sulphathiazole crystals has been conducted. In situ concentration monitoring was carried out using ATR-FTIR spectroscopy and the size and shape of the crystals produced were measured by automated image analysis. The simultaneous application of these two modern analytical methods was found to provide useful information for the thorough study of the causes and consequences of different crystallization conditions on the outcome of the process. The results obtained in this study showed a significant increase in the average crystal size with decreasing cooling rate when linear cooling profiles where applied. The average size of the sulphathiazole crystals produced by using the programmed cooling was found to be slightly smaller than the size of the crystals obtained by using linear or natural cooling profiles with the same batch time. The repeatability of the experimental procedure was considered to be good since the relative standard deviations between the results of the repeated batches were rather small in all cases. The variations observed in the properties of the crystals produced could be easily and comprehensively explained by the differences observed in the concentration profiles.