Abstract: The present invention provides a crystallization unit including a solvent reservoir in gaseous communication with a plurality of chambers. Preferably, a plurality of crystallization units are arrayed in the form of a crystallization tray. A first preferred embodiment of the present invention provides a crystallization tray 10 including a rectangular array of crystallization units 26. Each crystallization unit includes a central reservoir 28 and four drop chambers 32 arranged in a cruciform configuration around central reservoir 28. Each drop chamber 32 is connected to central reservoir 28 by a diffusion channel 30. Each drop chamber 32 also includes a shoulder which is capable of supporting a cover slip from which a drop of solution, containing the substance to be crystallized, can be suspended.