Generator systems realizing a dew-point temperature scale were compared using a chilled-mirror hygrometer. The comparison was carried out by the laboratories maintaining the national standards for humidity in Denmark, Finland, Poland, the Russian Federation, Switzerland, Turkey and the United Kingdom. The Centre for Metrology and Accreditation (MIKES, Finland) co-ordinated the project and acted as the pilot laboratory. The project was carried out in several sub-loops to produce information about the stability of the transfer standard. To analyse the results, the comparison reference value (CRV) was calculated at each measurement point using a cluster method. The CRV was then compared with the results obtained by the participants and normalized errors (En) were calculated. Most of the results show good agreement. The measurements at the METAS, UME and VNIIM did not cover the whole range of the project.