The asymmetric unit of the title compound, C9H8O2, contains two independent mol-ecules, both of which are essentially planar (r.m.s. deviations = 0.0294 and 0.0284 Å). The C=C double bond is in an E conformation and the vinyl-aldehyde groups adopt extended conformations. In the crystal, mol-ecules are linked by O-H⋯O hydrogen bonds, forming infinite chains parallel to [101].