Nucleation and metastability in crystallization of organic substances in organic solvents have been investigated. The focus was set on the effect of the thermal history of the solution on the metastability. Vanillin and ethyl vanillin were used as model substances. The metastable zone was determined in mixtures of water-alcohol and water-acetonitrile. The occurrence of polymorphism of vanillin due to temperature pre-treatment was also studied in some experiments. The measurements show that the metastable zone width is considerably affected by the extent of heating above the saturation temperature of the solution. The influence of overheating was found to be different for the various systems, and seemed to increase with the content of water in the solvent. A metastable phase of vanillin appeared in some solutions. The results indicate a preservation of structure "memory" on dissolution of the precipitate. In addition to these macroscopic experiments, attempts were made to examine the supersaturated and undersaturated solutions by small angle neutron scattering (SANS) studies. Studies of vanillin in 2-propanol-D2O mixture using small angle neutron scattering indicate the existence of aggregates in the solution prior to crystallization. The average size of the aggregates seemed to increase with decreasing temperature of the solution.