A new layered perovskite compound Ba-2(Cu0.5Na0.5)CO5 has been synthesised in the form of a pure sintered polycrystal and crystals of sub millimetre size that have been grown by solidification of stoichiometric melts. The structure corresponds to a distorted double perovskite with a tetragonal cell (s.g. P4/mmm, a=4.135(1), c=7.712(1) Angstrom) with alternating (Cu,Na)-O-2 and C-O-2 planes. The refinement of the atomic occupancies confirms the results of oxygen and carbon estimations and indicates that the average valence of the Cu ions must be close to 3. In the structural model we propose, the material lias a domain structure determined by the occurrence of exchange disorder in the cation sequence of the Cu,Na-O-2 planes.