This thesis describes the development of an effusive molecular beam apparatus, which allows the dosing of gases, liquids and solids. The apparatus was designed to adsorb precise and reproducible doses to a defined area on metal oxide samples, which is required in Thermal Programmed Desorption and other surface chemistry experiments in the Machine for Reactivity Studies. The theoretical profile of the molecular beam has a core diameter of 3.5 mm and a standard core pressure of 4x10e-8 mbar, while the background pressure of the beam is 4 orders of magnitude lower. The design and the construction of the apparatus is described. The beam profile was experimentally confirmed and core pressures between 1.7x10e-8 mbar and 2.9x10e-6 mbar were measured.