The title compound, (7R,8S)-7,8-dihydroxy-3,7-dimethyl-6-oxo-7,8-dihydro-6H-isochromene-5-carbaldehyde, C(12)H(12)O(5), is a trans-vicinal diol. Of the two fused rings, which lie approximately in the same plane, the pyran ring is almost perfectly planar, while the cyclohexenone ring adopts a slightly distorted half-chair conformation. The crystal packing is dictated by two strong intermolecular O-H.O interactions, one involving hydroxy and keto groups, the other involving two hydroxy groups. Molecules are linked together through twofold axes, forming zigzag ribbons extended along the a axis.