A key process in the shelf sea pumping of CO2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas. Here, we present measurements of profiles of the rate of dissipation of turbulent kinetic energy ($varepsilon$), from which vertical mixing is inferred, for several locations in the summer stratified zones of the NW European shelf seas. At both sites we find significant thermocline mixing, with ~20% of the observed turbulent dissipation taking place within the thermocline. We then present evidence to demonstrate that the thermocline mixing is a result of shear instability within the thermocline. This is contrary to the conventional view that vertical mixing in shelf seas is largely driven by boundary shear stresses.