In the title compound, C(9)H(11)NO·0.5H(2)O, the asymmetric unit contains two similar mol-ecules of 5,6,7,8-tetra-hydro-quinoline 1-oxide and one water mol-ecule. The water mol-ecule links the two O atoms of both independent N-oxides into dimers via O-H⋯O hydrogen bonds, forming a three-dimensional network along [101], which is additionally stabilized by weak C-H⋯O inter-molecular inter-actions. In each mol-ecule, the saturated six-membered rings exist in a conformation inter-mediate between a half-chair and sofa.