We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS's metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La-Nis. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni-5 metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.