The crystal structure of zinc 8-hydroxyquinolinate dihydrate, Zn(C~H6ON)~. 2H~O, has been determined by single-crystal methods. The unit cell is monoclinic with a = 11.28, b = 5.42, c = 13.16 A, fl = 106 ° 18'. The space group is P21/a-C~a and there are two molecules per unit cell.Atomic positions were determined by electron-density projections and were refined by a threedimensional Fourier electron-density synthesis and by the method of least squares. Structure factors were obtained from visually estimated intensities on Weissenberg photographs taken with Cu Kc~ radiation.The molecule, with the exception of the water molecules, is essentially planar. Including the water molecules, there is a sort of distorted octahedral arrangement of bonds around the central zinc ion.Zn-O = 2.05 A, Zn-N = 2.06/~, Zn-OH 2 ----2.27 /~.