The methane enrichment with hydrogen amounts can improve environmental and mechanical performances of an ICE (internal combustion engine). The paper presents data resulting by the experimental campaign carried out over 42 days on a demo commercial CHP (combined heat and power) (60 M-el) fuelled with hydrogen enriched methane blends. The tests have been carried out at partial load 55 kW(el) with hydrogen percentages of 0%, 5% and 8%. In order to evaluate the CHP energy consumption and environmental performance, the analysis was conducted for 336 h, using a continuous flue gas analyzer and two mass flow metres for hydrogen and methane. The aim of this paper is to demonstrate how the use of the hydrogen enrichment in a CHP plant represents a valid alternative to the same commercial energy systems, regarding the CO and CO2 emissions avoidance. The data analysis shows that modifying engine settings and increasing hydrogen amount significant energy effects occur. Consequently, the CHP fuelled with mixtures provides a lower thermal power, penalizing the heat recovery efficiency. The energy losses are due to unrecovered heat which is wasted like sensible heat to the stack. This alternative feeding system was developed in order to assess the energy penalty related to this carbon avoidance technique. (C) 2013 Elsevier Ltd. All rights reserved.