In the structure of the phthalide form of 2-acetylbenzoic acid, C9H8O3, there is a single type of hydrogen bond. Each molecule donates one and accepts one hydrogen bond. The hydrogen-bonded molecules form sets of puckered ribbons running along the c direction which are not crosslinked to each other. The dihedral angle between the planes of adjacent molecules along a hydrogen-bonded ribbon is 77.4 (1) °.