•M06-2X/6-311++G(d,p) level calculations on structures and tautomeric stabilities of 2-phenylpyrazolo4,3-cquinolin-3-ones.•X-ray crystallographic study of 2-phenylpyrazolo4,3-cquinolin-3-one monohydrates.•Supramolecular arrangements of 2-phenylpyrazolo4,3-cquinolin-3-one monohydrates. A single crystal X-ray diffraction and theoretical study has been carried out on mono hydrates of three 2H-pyrazolo4,3-cquinolin-3(5H)-one derivatives, namely 6-methyl-2-phenylpyrazolo4,3-cquinolin-3-one, 3, 6-methyl-2-(4-chlorophenyl)pyrazolo4,3-cquinolin-3-one, 4, and 8-methyl-2-(4-nitrophenyl)pyrazolo4,3-cquinolin-3-one, 5. The monohydrates were obtained on recrystallization from moist solvents. While there are three tautomeric forms possible for such pyrazolo4,3-cquinolin-3-one molecules, the sole form isolated in the solid (X)⋅(H2O) (X=3, 4 and 5) compounds was the quinoloid form – the one calculated to be the most stable at the M06-2X/6-311++G(d,p) level of theory. Excellent agreement was found between the calculated and X-ray determined structures. Molecule 5 in (5)⋅(H2O) is very near planar while both molecules 3 and 4 in their respective hydrates are much less so as a consequence of angles about 24° between the two aromatic rings. In each hydrate, the pyrazolo4,3-cquinolin-3-one molecule is bonded to three water molecules and each water molecule is likewise H-bonded to three pyrazolo4,3-cquinolin-3-one molecules. While the water molecules are H-bonded to 3 and 4 via the pyridinyl N and 2x the carbonyl O atoms, in (5)⋅(H2O) the H-bonds are to pyridinyl N, carbonyl O and a nitro O atoms. Calculations indicated that the found arrangement in (5)⋅(H2O) is more stable than one using the connections as found in (3)⋅(H2O) and (4)⋅(H2O). While each of the hydrates possess strong NH⋯O and OH⋯O hydrogen bonds, and weaker CH⋯π and π⋯π interactions, the supramolecular arrays are very different.