Abstract: Chemical synthesis apparatus includes a differential pumping device mounted in an evacuable container between an evaporant source providing a first chemical constituent and the reaction region. The differential pumping can be provided by apertured diaphragms or hollow cones, which may be cooled, mounted directly above the source which is thereby maintained at a lower pressure than the reaction region. The source may be an electron beam source or a resistively heated source arranged to direct the evaporant upwards into the reaction region and a second chemical constituent may be introduced into the reaction region. The invention also provides a method of chemical synthesis in which differential pumping is carried out between the reaction region and the source of one of the constituents.