Biologically produced mixtures of H-2 and CO2 (biohydrogen) from processes such as dark fermentation or photo-fermentation are versatile feedstocks which can potentially be utilised in solid oxide cell (SOC) devices. In this work, solid oxide electrolysis of biohydrogen has been investigated for the first time and is compared directly with fuel cell mode utilisation. The performance and fuel processing of SOCs utilising biohydrogen have been characterised in greater detail than has been achieved previously through the use of experiments which combine electrochemical techniques with quadrupole mass spectrometry (QMS). The effects of fuel variability on SOC overpotentials and outputs have been established and it is shown that cell performance is not significantly affected provided the fuel composition stays within 40-60 vol% H-2. QMS measurements indicate H2O and CO production takes place in-situ via the reverse water-gas shift (RWGS) reaction. Electrical power production in fuel cell mode is predominantly through H-2 oxidation, whilst CO is converted in the WGS reaction to regenerate CO2 but does not contribute to electrical power production. In electrolysis mode, CO is produced simultaneously through electrochemical CO2 reduction and the RWGS reaction; H2O is electrochemically reduced to regenerate H-2. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.