Long-term operational tests under direct biogas feeds of intermediate and high temperature solid oxide fuel cells (SOFCs) with Ni-cermet anodes, based respectively on gadolinia doped ceria (GDC) and yttria stabilized zirconia (YSZ) solid electrolytes are presented. Both biogas SOFCs seem to operate stably in a variety of biogas compositions, including poor-, equimolar- and rich-quality biogases, for a long period of time (for about 200 h at each condition, more than 600 h total operation for each cell). The only exception concerns the case of the intermediate temperature SOFC, when operated with poor biogas feed (i.e., CH4/CO2<1), where a systematic attenuation of cell performance is observed, resulting practically to its deactivation after similar to 100 h. Details on cells output characteristics under all operating conditions are presented and discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.