The results of an X-ray diffraction study of the decomposition process occurring in the K₂NiF₄-type aluminate LaCaAlO₄are reported. The experimental data concern two small single crystals heated for 1 and 10 h at 1200°C, respectively. As a final result, the presence of LaAlO₃perovskite and CaO is unambiguously observed. After 1 h of heating, several intermediate phases can be identified as small-size LaAlO₃-like microprecipitates. Well-defined orientation phenomena with respect to the matrix LaCaAlO₄are evidenced for these phases and for CaO as well. On the basis of the variation of the perovskite cell volume of the intermediate phases, the decomposition process is tentatively modeled. In this respect, a tetragonal phase is considered as the precursor of LaAlO₃perovskite.