We have investigated by high-resolution transmission electron microscopy SrTiO3/SrRuO3 bilayer films epitaxially grown on LaAlO3 substrate. At the SrRuO3/LaAlO3 interface we frequently observe dissociated superdislocations in connection with Ruddlesden-Popper-type planar defects. These superdislocations can contribute to misfit relaxation equivalent to a perfect misfit dislocation with Burgers vector a <1 1 0>. The formation and the geometry of this defect arrangement is discussed in detail. The Sr-rich Ruddlesden-Popper-defect can be introduced by composition fluctuations during film deposition. It can be concluded that it can act as a source for the generation of misfit dislocations. (C) 2002 Elsevier Science B.V. All rights reserved.