The dynamics of unstart of a floor-mounted inlet/isolator model in a Mach 5 flow are investigated experimentally. The inlet section contains a 6-deg compression ramp, and the isolator is a rectangular straight duct that is 25.4-mm high by 50.8-mm wide by 242.3-mm long. Measurements made include 8-kHz schlieren imaging and simultaneous fast-response wall pressures along the length of the inlet/isolator. Unstart is initiated by deflecting a flap at the downstream end of the isolator. The shock system, induced by unstart, initially propagates upstream through the isolator at a velocity of about 35 m/s (in the laboratory frame of reference), then decelerates to about 20 m/s near the isolator entrance, and then accelerates to a velocity of about 74 m/s within the inlet. Throughout the isolator, unstart is seen to be strongly associated with boundary-layer separation. Once the inlet has unstarted, a high-amplitude oscillatory (periodic) unstarted flow ensues, for which the oscillation frequency is about 124 Hz. However, under some conditions, an 84-Hz oscillatory unstarted flow mode, with lower pressure fluctuations, is observed. Under other conditions, a nonoscillatory unstarted flow, with much lower pressure fluctuations, is observed.