We have fabricated epitaxial La1.2Sr1.8Mn1.7Ru0.3O7 thin films with n=2 Ruddlesden-Popper (RP) structure on TiO2-terminated and SrO-buffered SrTiO3(0 0 1) single-crystal Substrates. It is shown that even small deviation from the narrow growth window results in the nucleation of (AO)(2) double layers in random directions and the loss of the desired structure. The atomic layer ordering at the interface between the thin films and SrTiO3 substrates was Studied by high-pressure, in situ reflection high-energy electron diffraction and transmission electron microscopy. It is demonstrated that growth on TiO2-terminated SrTiO3 commences with the nucleation of two perovskite sub-units before the first (AO)(2). double-layer forms. To obtain a sharp RP/SrTiO3 interface structure, it is necessary to buffer the TiO2-terminated SrTiO3 substrate with a sub-unit cell SrO layer before the deposition of the RP film. (C) 2008 Published by Elsevier B.V.