A crystalline material sufficiently pure for use in pharmaceuticals may be made by forming a saturated solution of the material, changing the temperature of the solution so it becomes supersaturated, and subjecting the solution to irradiation by high intensity ultrasound, the frequency of the ultrasound being scanned over a range of frequencies. For example the ultrasound may be varied between 19.5 and 20.5 kHz, and this variation may be sinusoidal. Preferably the ultrasound is provided only briefly, say for less than 5 s, before allowing the solution to cool gradually without further irradiation. The ultrasound may be applied using a vessel with an array of ultrasonic transducers attached to a wall, so each transducer radiates no more than 3 W/cm.sup.2 yet the power dissipation within the vessel is between 25 and 150 W/litre. This method can reduce the metastable zone width to less than 10 K. It is applicable in particular to aspartame.