This paper reports on a series of steam injection experiments on a Turbec 1100 microturbine. Combined Heat and Power (CHP) systems, such as the considered 1100 microturbine, use one single primary fuel to simultaneously produce electric and thermal power. In doing so, they realize significant energy savings compared to conventional schemes of separated production. However, a reduction in the demand for heat (e.g. in summertime) will force this type of units to shutdown. This significantly reduces the amount of operating hours and has a severe negative impact on the net present value of such CHP investment projects. The aim of this paper is to investigate and demonstrate the effects of steam injection in the compressor outlet of a microturbine operating under reduced heat demand conditions, in order to keep the unit running. The necessary steam can be auto-raised with heat available in the turbine exhaust downstream of the recuperator. Such an injection will keep the unit running and thus avoid a forced shutdown. Furthermore, it is expected that the electric efficiency will rise and that the power production will become more economically viable as a result of the increasing operating hours. This paper reports on the influence of steam injection on the electrical efficiency and shaft speed of a 1100 unit. ASPEN (R) simulations of the behavior of the CHP unit are also presented. These simulations predicted a 2.2% rise in electric efficiency at nominal electrical output when 5% of the mass flow rate of air is replaced by steam. The steam injection experiments resulted in stable runs of the unit, a predicted reduction in shaft speed and increasing electrical efficiency. Validation of the ASPEN (R) simulations against the experimental data revealed the necessity for a more accurate determination of the air mass flow rate and more precise compressor characteristics. (C) 2012 Elsevier Ltd. All rights reserved.