The use of a vapor source (10) in an isothermal system (8) to control the crystallization or recrystallization of a sample of solid material (9) which is initially at least partially amorphous and/or at least partially crystalline and contained within the system (8), the vapor source (10) including at least one solvent for crystallizing the solid material, the vapor diffusion (14) of which leads to the crystallization or recrystallization of the sample (9), the vapor source (10) being such that the quantities of heat exchanged within the system during the crystallization or recrystallization of the sample (9) other than the heat of crystallization or recrystallization of the sample are less than approximately 10%, particularly 5%, and advantageously 1% of the heat of crystallization or recrystallization of the sample. The vapor source (10) is preferably a pure solvent or a solvent mixture in which no solute is dissolved.