The crystallization process of vanillin in a 1-propanol/water system was investigated. It was found that liquid-liquid phase separation, also termed, oiling out, will happen under some conditions To fully; understand this process, the oiling out as well as the following nucleation and crystal growth of this compound, were investigated by using series of in situ tools such as FBRM, ATR-FTIR, and PVM. The phase diagram, including the solubility curve and the liquid-liquid separation curves, was determined experimentally. Meanwhile, the influence of oiling out on the properties Of the final, crystalline product was analyzed. It was proposed that crystallization mechanisms of vanillin might be significantly different depending on the position of the crystallization profile in the phase diagram. By using the fundamental data of phase diagram, control and optimization of solution crystallization of vanillin could be achieved to avoid or suppress the oiling out and to get products with high purity and ideal crystalline properties.