Lanthanum nickelates: La2NiO4+delta, La3N2O7-delta, La4Ni3O10-delta and LaNiO3-delta the members of Ruddlesden-Popper series Lan+1NinO3n+1 were prepared using citrate route. Dissolution enthalpies of complex oxides as well as a number of subsidiary substances were measured by means of Calvet calorimeter in 1 M solution of hydrochloric acid at 25 degrees C. The dissolution scheme of complex oxides in hydrochloric acid was proposed and enthalpies of formation of the complex oxides from binary oxides were calculated considering oxygen nonstoichiometry of these substances. Enthalpies of step-by-step oxidation were evaluated. Partial enthalpy contribution of LaO layers was calculated endothermic equals to 30.9 J/mol while partial enthalpy contribution of perovskite LaNiO3 layers was negative equals to -97.0 J/mol. Enthalpy of fort-nation of any complex oxide of Ruddlesden-Popper series fits very well to the linear regression based on these values. (C) 2006 Elsevier B.V. All rights reserved.