The results of the isothermal expansion as a function of oxygen partial pressure P-O2 and temperature are first presented for the perovskite-type undoped cobaltite LaCoO3-delta. The modeling of the chemical expansion of this oxygen deficient perovskite on the basis of its defect structure is carried out. The chemical expansion model based on the relative change of the mean ionic radius is evolved. Within the framework of the model electrons and electron holes are assumed to be localized on cobalt sites in LaCoO3-delta. The corresponding equations for the concentration of all ionic species depending on oxygen nonstoichiometry delta enable to derive a model expression, which is used for the computation of the LaCoO3-delta chemical expansion versus oxygen nonstoichiometry. The experimental data on the chemical expansion of LaCoO3-delta, and those calculated on the basis of the model proposed are shown to coincide with each other very good. Such coincidence implies explicitly that the isothermal expansion of LaCoO3-delta observed with oxygen nonstoichiometry growth at given temperature is solely caused by the mean ionic radius increase. The latter occurs due to the partial substitution of "large" Co(n - 1)-cation for smaller Con+ cation as a result of the reduction. On the other hand, aforementioned agreement can be regarded as an additional evidence of the localized nature of the electronic defects in the undoped lanthanum cobaltite. (C) 2008 Elsevier B.V. All rights reserved.