In order to investigate the solid oxide fuel cell nickel-yttria stabilized zirconia (Ni-YSZ) anode degradation mechanism due to poisoning by Octamethylcyclotetrasiloxane (D4), four different experiments utilizing H₂+H₂O, H₂+H₂O+D4, H₂+D4 and H₂+CO+D4 as fuels at 750 °C are conducted in this study. The electrochemical characterization and morphology results are analyzed and compared for anode degradation phenomenon. The results contradict the previously proposed degradation mechanism as the experimental results show that water can inhibit the silicon deposition and anode degradation. Based on the experimental results in this study and previous studies about siloxane deposition on metal oxides, a new anode degradation mechanism is proposed. The proposed degradation mechanism for siloxane is due to siloxane adsorption resulting in silicon deposition and carbon deposition after siloxane decomposition.