In this study, the liquid liquid phase separation and crystallization of vanillin from 1-propanol/water solution were investigated. It was found that water could contribute to the improvement of vanillin solubility in 1-propanol/water solution, but the solubility of vanillin decreased with the increasing water/ 1-propanol ratios, when the water content was over 30 wt.%. High water fraction in 1-propanol/water solvents will result in the liquid liquid phase separation in the crystallization process, and the liquid liquid phase separation dominated the process before the nucleation of crystals during the crystallization of vanillin. A ternary phase diagram (vanillin/1-propanol/water) including the solubility curves and the liquid liquid phase separation curves was measured and could, in certain cases, clearly illustrate the liquid liquid phase separation and crystallization process of vanillin in 1-propanol/water solutions. As shown in the ternary phase diagram, liquid liquid phase separation under high initial vanillin concentration results in low purity of vanillin crystals due to the inhibited crystallization process by liquid liquid phase separation. Partial phase immiscibility was a function of temperature and the range of immiscibility decreased with increasing temperature. Moreover, nucleation and crystal growth of vanillin in the presence of liquid liquid phase separation were investigated; process parameters such as the composition of solvent, cooling profiles and seeding were optimized to avoid liquid liquid phase separation and thus increase the purity of crystallization products. (c) 2015 Elsevier B.V. All rights reserved.