The electrochemical behaviour of calcium in neutral thionyl chloride solutions containing FeCl3, AlCl3, CaCl2 and CaO was investigated using pulse galvanostatic and ac techniques. In all systems the calcium electrode is completely covered by a passivating layer consisting of CaCl2 which serves as a SEI. The SEI is probably an anionic conductor. The ionic transport number (ti) of the SEI under OCV conditions was calculated from the ratio: OCV/thermodynamic EMF. It is about 0.9 for AlCl3-based electrolytes and about 0.8 for FeCl3-based electrolytes. Immediately after laoding the cell at about 10mA cmt-2 its OCV reaches 3.7 V indicating that ti reaches unity and the OCV of the cell equals the thermodynamic value. The electrode reaction resistance and the apparent thickness of the SEI varies with type and concentration of the electrolyte and with storage time. When an aged electrode is anodically dissolved, the reaction resistance drops by three orders of magnitude, and the SEI-resistivity drops by two orders of magnitude. The mechanism responsible for the "diode" behaviour of the calcium-SEI electrode in thionyl chloride is discussed.