A dynamic SOEC (Solid Oxide Electrolysis Cell) model is proposed to investigate the transient response and steady performance of a planar SOEC. Three representative types of continuity equation expressions are systematically compared for the simulation of source terms introduced by electrochemical reactions. For the conservative form of continuity equation (Type A), reasonable predictions at both sides of cathode and anode cannot be achieved, as the diffusion effect is neglected. The non-conservative form of continuity equation (Type B) can obtain reasonable prediction for the cathode side but poor prediction for the anode side. The Type C of continuity equation, newly proposed by the authors for modeling the SOECs, is based on the law of volume conservation. It could achieve the volume increment (oxygen produced) at the anode compartment and good agreements with the analytical ones. It is also found that continuity equations significantly influence the fluid flow and mass transport, whereas their effects on the electrical characteristics are negligible when the global current density is not high. (C) 2017 Elsevier Ltd. All rights reserved.