Energy generation from renewable sources in the power sector keeps constantly increasing. This raises the demand for fast and flexible large-scale storage technologies. Steam generation via stoichiometric combustion of hydrogen and oxygen within a steam cycle is a promising way to recombine both gases, which can be generated by electrolysis utilizing excess renewable energy. At the same time, this technology could provide balancing and spinning network reserves. A crucial parameter of this approach is the combustion efficiency, since residual hydrogen or oxygen can damage downstream components of the power plant steam cycle. The current paper investigates the combustion of hydrogen and oxygen under steam diluted conditions. Flow field, mixing, flame types and combustion efficiency are assessed. The combustion efficiency measurement is very challenging in this case, as the combustor products consist mostly of pure steam and cannot be dried for conventional gas analysis. This is solved by an in-situ measurement method to quantify the combustion efficiency. Initial results of this approach are also presented in the current work. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.