We report an experimental investigation of the separating/reattaching flow over a descending ramp with a 25 • expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the Turbulent/Non-Turbulent Interface (TNTI) and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasize the multiscale nature of mass entrainment through the separated shear layer. The recirculation length L R , step height h and free stream velocity U ∞ are the dominant scales that organise the separated flow (and related large scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large scale mass entrainment is driven by incoming boundary layer properties, since L R scales with Re θ , and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.