1Lanthanides are an interesting class of ions for applications in a variety of light-emitting materials because of their rich energy level structure. For many applications, control over the transition rates between specific energy levels is desirable, to be able to tune the emission output and the absorption strength. One method to achieve this is by bringing lanthanide ions close to a metal nanoparticle and use plasmon coupling to modify transition rates. In this letter we present measurements on a model system consisting of silica-coated gold nanoparticles for which lanthanide complexes have been incorporated in an amphiphilic layer surrounding the silica. We varied the thickness of the silica layer between 7.6 and 19.1 nm and incorporated the Eu(TTA)3 complex in the surrounding amphiphilic layer. For the 5D0 emission of Eu3+ we measured almost no decay rate enhancement with a 20 nm silica shell surrounding the Au particle, while for the thinnest shells we measured a decay rate enhancement up to a factor of five. Comparison with classical electromagnetic theory shows a good agreement between the experimentally observed enhancement and theory.