The types, form and purity of materials used for neutron metrology in the European Community countries has been reviewed by the Monitor Materials Sub-Group of the European Working Group on Reactor Dosimetry. The requirements of dosimetry techniques used in operational reactors and in associated research and development programs using irradiation rigs are discussed. The specifications for purity and problems associated with the characterization of these materials that have arisen during the dosimetry material production program are reviewed.