X-ray diffraction and transmission electron microscopy were applied to investigate a sol-gel synthetic process for the mixed oxygen ion and electron conductor La2NiO4+delta with a K2NiF4 structure type. The development of the La2NiO4+delta is elucidated considering the influence of calcination temperatures and dwell times. Following the thermal decomposition of nitrate and organic precursors in an intermediate step, the lanthanum nickel oxide is obtained after a short dwell time above 750 degrees C. This occurs by the transformation of an ultrafinely dispersed powder consisting of lanthanum oxycarbonate, lanthanum oxide, and nickel oxide. The pure La2NiO4+delta phase was obtained by similar solid-state reactions between nanocrystalline powder particles at just 950 degrees C.