Crystals of 2-propenoic acid are orthorhombic, space group Ibam, with a 9.952(2), b 11.767(2), c 6.206(2) .ANG.; Z = 8, dc = 1.317; R = 0.040, Rw(F2) = 0.118 for 1198 reflections. The compd. belongs to the family of a,b-unsatd. carboxylic acids having a Z conformation. The packing is characterized by layers of O-H...O H-bonded dimers.