The modeling of the modulation of the X-ray diffuse scattering background is applied to get evidence of local ordering phenomena of La³⁺ and Ca²⁺ in the K₂NiF₄ type aluminate LaCaAlO₄. Such short range ordering phenomena play a precursor role in the demixion of the intergrowth structure of LaCaAlO₄ in LaAlO₃ and CaO, the perovskite and rock-salt type parent oxides, respectively. As the first stage of formation of twinned LaAlO₃ precipitates with the matrix, a La³⁺ enrichment occurs in the {113} planes of the tetragonal cell of LaCaAlO₄. The tendency to get simultaneously a short range ordering of the first nine neighbours and a clustering of the 8 second neighbours of Ca²⁺ accounts well for the formation of `LaO<sub>3</sub>' compact planes, ensuring the progressive coming out of the perovskite structure. In comparison with LaCaAlO<sub>4</sub>, no local ordering is detected in the stable aluminate NdCaAlO<sub>4</sub>, as in YCaAlO<sub>4</sub> a slight tendency to get a short range ordering of both the first and second neighbouts results in the formation of mixed `Y,CaO₃' compact planes. The local ordering phenomena prove not to be contradictory to the overall randomness of the cationic distribution which is the usual crystal-chemical property of the A³⁺A′²⁺AlO₄ aluminates.