Experimental (NMR, X-ray and DSC) and theoretical studies [DFT B3LYP/6-311++G (d,p)] have permitted to establish the structure of the main tautomeric form of 8-hydroxycarbostyril 1. In the gas phase two tautomers, 8-hydroxyquinolin-2(1H)-one 1a and 2,8-quinolinediol 1d, show similar stabilities. In solid state two polymorphs of 1a have been isolated and their structures elucidated, polymorph A which crystallizes in Pccn orthorhombic space group and polymorph B in the P21/c monoclinic space group. The arrangement of molecules in both structures is characterized by intermolecular N–H⋯O and O–H⋯O hydrogen bonds.