The crystal structure of Yb(C5H7O2)3(H2O), tris(acetylacetonato)aquoytterbium(III), has been determined from threedimensional X-ray diffraction data collected by counter methods. A total of 2992 independent reflections, for which the intensity exceeded 1.07 times the background, were recorded as observed. Four molecules crystallize in a triclinic unit cell of symmetry P1 and dimensions a = 13.01 Å, b = 18.33 Å, c = 8.32 Å, α = 100.40°, β = 102.12°, and γ = 105.19°. The calculated and observed densities are 1.72 and 1.74 g cm-3, respectively. The structure has been refined by the method of least squares to a value of 0.076 for the conventional R factor. The two crystallographically independent ytterbium ions are seven-coordinate, each being bonded to three acetylacetonate groups and one water molecule. A hydrogen bond links the independent molecules in pairs. The coordination polyhedra formed about the independent ytterbium ions may both be described as capped trigonal prisms. © 1969, American Chemical Society. All rights reserved.