A single crystal X-ray diffraction study was performed on the compounds [Ca2Al(OH)6]·[Cl∼0.5(CO3)∼0.25·∼2.25H2O] belonging to the cementitious AFm family of general formulae [Ca2Al(OH)6]·[X·nH2O], where X is a monovalent anion, or half a divalent anion. The so-called chloro-carboaluminate compound crystallizes in the rhombohedralspace group with: a=5.7400(4) Å, c=46.7402(4) Å, V=1333.7(2) Å3, Dx=2.054 g/cm−3, and Z=6. Refinement of 283 independent reflections led to a residual R factor of 0.020. Chloride and carbonate anions are statistically distributed into the same crystallographic site (6a Wyckoff site). The structure of the chloro-carboaluminate compound corresponds to the high temperature phase of Friedel's salt, the equivalent chloride AFm compound with composition [Ca2Al(OH)6]·[Cl·2H2O]. Three powdered samples of composition [Ca2Al(OH)6]·[Cl1−x(CO3)x/2·∼2.25H2O], with x=0.25, 0.5, and 0.75, were synthesized and characterized in order to investigate the structure transition of Friedel's salt (from the rhombohedral HT-structure to the monoclinic LT-structure) versus the carbonate substitution level. Whereas a structure transition is observed around 35°C for the carbonate-free Friedel's salt, sample with x=0.25 shows a similar structure transition around −15°C. The two other samples with x=0.5 and 0.75, multiphase, exhibit a more complicated thermal behavior.