Abstract Hydrate formation of haloacetic acids CClnF3-nCOOH (n = 1, 2, 3) has been studied by DTA and temperature-dependent X-ray powder diffraction, and evidence obtained for five phases, all melting at temperatures below ambient. The hydrates have been confirmed and further characterized by their crystal structures at -150 °C. Three lower hydrates, CCl2FCOOH· 0.5H2O (space group P21/c and Z = 8 formula unites per unit cell), CClF2COOHH2O (P21/c, Z = 4), and CClF2COOH ·4H2O (P1̄, Z = 2), have molecular structures with the acid and water molecules hydrogen-bonded in two-dimensional arrays. The structures of the remaining hydrates, CCl2FCOOH·6H2O and CCl3COOH · 6H2O (similar, but not isotypic, both P1̄ and Z = 2), are ionic, as (H5O2 +)(CX3COO-) · 4H2O , and three-dimensional. The neutral water molecules are hydrogen-bonded in ice-like layers rare in crystal chemistry. Also, CCl2FCOOH is established only as the second acid of which a lower hydrate is molecular and a higher one ionic.