Scientifically sound certification protocols, to assess the performance of elemental mercury (Hg⁰) and oxidised mercury (Hg^II) gas generators in the form of formally accepted documentary standards, are of fundamental importance to guarantee the accuracy and comparability of the mercury measurement results in gas emission sources and in the atmosphere in Europe and globally. The certification protocols contribute to establishing a traceability chain from a primary mercury gas standard to measurement data obtained in gas emission sources and in the atmosphere. Standardisation committee CENTC264/WG8 has already formulated a new work item on the certification protocol to transform the protocols developed in the project into European standards.