Ab-initio studies showed that large Seebeck coefficients could be achieved in La2NiO4+delta layered perovskite oxide by tuning its oxygen content. We have prepared La2NiO4+delta by soft-chemistry procedures and annealed the samples under different conditions (atmospheres and temperatures), yielding samples with extra oxygen content (delta) in the 0.09-0.30 range. The exact stoichiometry of the samples was determined by iodometry and by a neutron powder diffraction analysis for a selected sample. Thermoelectric properties were studied and the thermopower was measured in the differently treated samples. Thermopower as high as S= 250 mu V K-1 at T= 100 K and low thermal conductivity values as low as 0.8 W m(-1)K(-1) at room temperature have been obtained in La2NiO4+delta polycrystalline samples with delta= 0.094. (C) 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of the Conference Committee Members of 14th EUROPEAN CONFERENCE ON THERMOELECTRICS.