This paper focuses on non-isothermal transient flow in mixed hydrogen-natural gas pipelines. The effect of hydrogen injection into natural gas pipelines has been investigated in particular the pressure and temperature conditions, Joule-Thomson effect, linepack and energy consumption of the compressor station. The gas flow is described by a set of partial differential equations resulting from the conservation of mass, momentum and energy. Real gas effects are determined by the predictive Soave-Redlich-Kwong group contribution method. The Yamal-Europe gas pipeline on Polish territory has been selected as case study. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.