The crystal structure of trichloroacetic acid (CCl3COOH) has been determined by application of direct methods of phase determination to single crystal neutron diffraction data. The structure is monoclinic (P21/c) with cell constants at 22°C: a=6.152(2) Å, b=9.705(4) Å, c=12.020(9) Å, β=124.29(5)°, Dc = 1.830 gcm−3,Dm = 1.81 g cm−3. The value of R(F) for 615 independent reflections is 0.057. There are four molecules, crystallizing as two centrosymmetric, hydrogen-bonded dimers in the unit cell. The bond lengths (corrected for rigid-body motion) are C–O, 1.311(4) Å; C=O, 1.216(4) Å, C–C, 1.546(4) Å, C–Cl (mean), 1.768(3) Å; O–H, 1.013(11) Å. The O··· O distance of the hydrogen bond is 2.666(6) Å, and the O–H···O angle is 176.5(5)°. A rigid-body analysis of the thermal motion indicates a librational amplitude of 10° approximately about the long axis of the dimer, smaller librations about the other two axes, and rms translational amplitudes of 0.20 Å in all directions. The apparent vibrational amplitude of the hydrogen atom along the hydrogen bond is 0.12 Å. This exceeds the value to be expected for a harmonic oscillator with the observed vibrational frequency; substantial anharmonicity is suggested. There is no evidence of disorder.