The size, shape, and interface Structure of poly(styrene-b-methyl methacrylate) block copolymer domains assembled oil ail epitaxial template are characterized with soft X-ray diffraction, The domain size and shape are deformed when the dimensions of the epitaxial template are incommensurate with the equilibrium dimensions of the block copolymer, producing sidewall angles in the range of 1-2 degrees (+/-0.2 degrees). The average width of the copolymer interface is (4.9 +/-0.1) nm. Comparison with mean-field theoretic predictions for the Structure of block copolymer interfaces Suggests a low-frequency variance in the copolymer interface position of 1.2 nm(2), or a low-frequency line-edge roughness of approximately 3 nm.