The structures of two phases of the title compound, C10H8N2O2.H2O, are reported. While the organic molecule has very nearly the same molecular geometry in the two phases, the hydrogen-bonding patterns are quite different. In each case, however, a three-dimensional network of hydrogen bonds is formed. In the  phase, six N-HO hydrogen bonds have donor-acceptor distances ranging from 2.681 (2) to 3.206 (2) Å, while three O-HO hydrogen bonds range from 2.837 (2) to 3.173 (2) Å, and in the  phase, four N-HO hydrogen bonds range from 2.668 (2) to 3.038 (2) Å, while two O-HO hydrogen bonds are 2.951 (3) and 2.959 (3) Å in length.