Provided are methods for preparing and using reaction vessels obtained or obtainable by 3D-printing methods, including a method for preparing a product compound, the method comprising the steps of: (i) providing a reaction vessel that is obtained by a 3-D printing method, wherein the reaction vessel has a reaction space; (ii) providing one or more reagents, optionally together with a catalyst or a solvent, for use in the synthesis of the product compound; and (iii) permitting the one or more reagents to react in the reaction space, optionally in the presence of the catalyst and the solvent, in the reaction vessel, thereby to form the product compound.