Single crystals of acetyltetralone grown from a solution containing a small amount of Cu(II) ions are studied by ESR. The corresponding g, ⁶³Cu magnetic and quadrupolar hyperfine tensors are obtained and are consistent with the trapping of a square planar copper(II) complex. X-ray diffraction shows that pure acetyltetralone crystallizes in the orthorhombic space group Pbca (a = 8.893 A, b = 20.301 A, c = 10.715 A). Comparison of the ESR eigenvectors with the bond directions obtained from this crystal structure study shows that the Cu(II) complex experiences some constraint from the crystal matrix. The present complex is a model for one of the complexation sites of tetracyclines.