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An account is given of transmission electron microscope studies of the crystal structure of Y2Cu2O5 discovered within samples of YBa2Cu3O7-x and Y1.85Sr0.15CuO4- delta ceramic compounds. Conventional and high-resolution electron microscope investigations confirmed that Y2Cu2O5 is isostructural with Ho2Cu2O5 and has a space group P21nb with lattice parameters a=1.247 nm, b=1.081 nm and c=0.3495 nm. The nature of planar defects discovered within the crystallites of Y2Cu2O5 is discussed. It is proposed that small amounts of barium that do not react to form a solid solution with Y2Cu2O5 segregate to the lines of defects. The displacement vector for the planar defects was found to be 1/2(014).