The shock dynamics in the isolator of a free-jet model scramjet is investigated during unstart at low enthalpy conditions. Mass injection is used to choke the combustor flow and cause unstart by simulating the blockage effect of combustion heat release. The model scramjet used has a constant area circular cross section and optically accessible isolator and combustor. A detailed characterization of the unstart process was provided by wall pressure measurements and high-speed (10 kHz) CO2 Rayleigh scattering imaging. In response to the backpressure rise induced by mass injection, the flow in the isolator separates, forming a shock-train. The flow in the combustor stays supersonic until choking occurs near the exit. Once the flow is choked, a pseudo-shock propagates upstream and reaches the injectors location, where it starts interacting with the isolator shock-train. Under particularly severe blockage conditions the shock system ultimately disgorges from the inlet causing unstart.