The title compound crystallises in space group Pca21 with two molecules in the asymmetric unit, the cell having dimensions a= 8·060, b= 4·195, c= 25·433 Å. The phases were determined from a diffractometer data by a multi-solution method and the structure refined by a least-squares procedure to R 0·067 for 968 independent reflections. The bond lengths are CC 1·175, 1·179; C–C 1·455, 1·453; C–CO2H 1·484, 1·506; CO 1·221, 1·214; C–OH 1·307, 1·259 Å. The C(:O)·C·CC system is syn-planar. The molecules are hydrogen-bonded (O–H 2·69 and 2·65 Å) in pseudo-centrosymmetric pairs. In the crystal there are no intermolecular ethynyl-H OC interactions; rather, the acetylenic; C–H bond points at the CC triple bond, [H CC(midpoint) 3·0 Å]. The C(sp3)–H bond is directed towards the carbonyl oxygen (C–H O 3·45, and H O 2·6 Å). But-3-ynoic acid also appears in a pseudo-orthorhombic form showing order-disorder. The relationship of the two forms is discussed.