The formation of carbon is a problem when operating solid-oxide fuel cells (SOFC) containing nickel/yttria-stabilized zirconia (Ni/YSZ) anodes with hydrocarbon fuels directly. Previous studies have examined the carbon formed ex-situ on Ni/YSZ anodes. In this study, we have examined the carbon formed in-situ under operating conditions, and investigated the effects of current density, time, and anode thickness on both the amount and type of carbon formed at 1073 K using temperature programmed oxidation and scanning electron microscopy. The carbon formed under operating conditions was more reactive and contained hydrogen compared to the carbon formed at open circuit potential that did not contain hydrogen. When the current density was increased less carbon was deposited, but the characteristics of the carbon did not change. With increasing time and/or anode thickness, the carbon deposits became more difficult to remove and decreased in hydrogen content. (c) 2008 Elsevier B.V. All rights reserved.