The X-ray diffraction method was used to study the thermal decomposition of single crystals of the complex aluminate LaCaAlO₄ which features heterovalent-stoichiometric isomorphism of the calcium and lanthanum cations. It is found that decomposition of the compound begins with surface and involves intermediate metastable phases (P′ and P″), whose existence is proved by a specific diffuse X-ray scattering. The final decomposition products CaO and LaAlO₃ are orientationally connected with the crystal matrix. A possible mechanism of the decomposition of LaCaAlO₄ and thermodynamic stability of such compounds are discussed.