An experimental investigation of the solidification of a fully developed turbulent internal flow due to external cooling of the tube wall was conducted. A region of unstable freezing behavior was located and bounded in terms of initial Reynolds number and a nondimensional temperature parameter for the given system. This region was observed over the span of Reynolds numbers from 4500 to 10150 and was defined to exist at values of the pertinent parameters between those which cause a smooth steady-state response and those which cause a smooth response resulting in complete tube blockage. An increase in upstream flow resistance was found to make the region of instability less sensitive to a change in coolant temperature at constant initial Reynolds number. System blockage was predicted in terms of steady-state freezing data at constant nondimensional temperature and various initial Reynolds numbers.