The crystal structure of tt-CaC12.4H20 was redetermined from three-dimensional X-ray data collected by a Nonius CAD-4 diffractometer with Mo Kc~ radiation. The compound crystallizes in the triclinic space group P1 with two formula units in the cell. Cell dimensions are a = 6.5932 (6), b = 6.3673 (8), c = 8.5606 (15) A, a= 97.83(1),fl= 93.50(1), 7= 110.58(1) ° , V= 331.02 A 3. The H atoms were located by difference syntheses. After least-squares refinement R = 0.033, R w = 0.038. The Ca coordination is seven and not six as considered in previous work. The structure consists of [CaC12.4H20] 2 units held together by hydrogen bonds.