We propose an improved version of the symmetric metal slot waveguides with a Kerr-type nonlinear di-electric core adding linear dielectric buffer layers between the metal regions and the core. Using a finite element method to compute the stationary nonlinear modes, we provide the full phase diagrams of its main TM modes as a function of total power, buffer layer and core thicknesses, that are more complex than the ones of the simple nonlinear metal slot. We show that these modes can exhibit spatial transitions towards specific modes of the new structure as a function of power. We also demonstrate that, for the main modes, the losses are reduced compared to the previous structures, and that they can now decrease with power. Finally, we describe the stability properties of the main stationary solutions using nonlinear FDTD simulations. Nonlinear plasmonic slot waveguides (NPSWs) have drawn attention in the last decade due to the strong light confinement in the nonlinear dielectric core ensured by the surrounding metal regions, and to their peculiar nonlinear effects [1–6]. Several applications have already been proposed for NPSWs [7, 8]. Nevertheless, the experimental observation of plasmon-soliton waves in these NPSWs is still lacking even if linear slot waveg-uides have already been fabricated [9]. Similarly to the case of the single nonlinear dielectric/metal interface structures [10, 11], the modes already studied in the simple NPSWs suffer from high losses that seriously limit the propagation length of the waves. In the present study, we propose and study an improved structure in which buffer linear dielectric layers are added between the nonlinear dielectric core and the two semi-infinite metal regions. The article is organized as follows. First, we describe the model and numerical method we use to study the stationary nonlinear waves in the improved NPSW we propose. Secondly, we describe the linear solutions of the new structures in order to classify the nonlinear solutions. Thirdly, we give the properties of the nonlinear stationary solutions and show that, for some linear parameter configurations, new modal spatial transitions as a function of power occur compared to the simple NPSW case. We provide full phase diagrams for the improved NPSW as a function of total power, buffer layer and core thicknesses. We also prove that the added buffer dielectric layers are able to reduce losses and allow them to decrease with power in most of the cases. Finally, using the FDTD method we study the stability properties of the main nonlinear solutions.