The process of formation of the Ruddlesden-Popper phases LnSr(2)Mn(2)O(7) (Ln = La, Nd, Gd) was investigated by means of X-ray diffraction, thermal analysis and microprobe analysis. Two parallel pathways were found for the reaction. The first one includes the formation of the intermediate K2NiF4-type compound by interaction of Sr7Mn4O15 and perovskite-type solid solution (Ln,Sr)MnO3, while in the second one the K2NiF4-type intermediate is formed during the reaction of the same perovskite-type solid solution with the corresponding rare earth oxide. This result gives an explanation for the formation of two phases with slightly different cationic compositions. The kinetics of the reaction was investigated for both mechanisms resulting in determination of their relative contribution to the final composition. Pure NdSr2Mn2O7 was obtained using a step-by-step process according to the first mechanism, and the magnetic properties of this phase were investigated. (C) 2012 Elsevier Ltd. All rights reserved.