[en] High temperature C-ray diffraction is used to investigate calcium uranate of theoretical formulation 'CaUO₃' which is obtained either by interaction of uranium dioxide and lime under vacuum or by 'CaUO₆' oxide reduction. This new phase of monoclinic symmetry between 20 and 1,500 deg. C becomes orthorhombic above 1,500 deg. C, and shows a linear expansion and a constant composition between 800 and 1,500 deg. C (U/O + Ca ≅ 1.5). The pseudo-cubic cell displays at 20 deg. C a 0.6 % linear increase with regard to the isomorphic compound 'Ca₃UO₆' which is related to U^VI -> U^IV substitution between the two distorted perovskites. (authors)