During the preparation of stoichiometric LaAlO3 type oxide from an aqueous solution of La and Al nitrate salts, using starch as template, a layered intermediate – disordered mixed lanthanum and aluminum hydroxycarbonate exists in the 300–700 °C calcination temperature range. According to calcination temperature a variable content of anions/OH is observed as well as changes in the coordination environment of La and Al cations are providing distortions in the structure. The nuclei of LaAlO3 appears within a mixed La-Al hydroxycarbonate matrix at ∼700 °C. This phase crystallizes at 900 °C after decomposition of the major part of bulk carbonates. The surface layer of perovskite particles formed by stacked nanocrystallites is enriched in La cations and contains residual hydroxyls and carbonates, which results in distortion of the local environment of both cations creating oxygen defect sites as well. Such a microheterogeneity is partly retained up to 1300 °C.