The mean and standard deviation of the differences between EPN and ERA Interim ZTD are 6.0 mm and 13.6 mm before 5 November 2006 and 5.0 mm and 12.4 mm, respectively, after that date. These results evidence the strong impact of the changes introduced in the GNSS processing in November 2006, in particular the adoption of new PCV maps. In the EPN products these changes introduce a bias of about 1 cm. Byun and Bar- Sever (2009) had reported similar, although smaller, biases between the IGS products generated before and after the adoption of the IGS standards for PCV modelling in November 2006. These results show that both U.Porto and IGS ZTD solutions are adequate for use in applications that require long term stability, while the EPN products prior to November 2006 require a suitable correction. Since this correction is mainly a bias, it can be estimated by comparison with a stable and uniform data source such as the ECMWF ERA Interim product.