Twenth-three blowdown tests were performed to evaluate pool dynamics effects in the Mark II containment geometry. The tests utilized the steam generator and drywell of the Pressure Suppression Test Facility (PSTF) and the Temporary Tall Test Tank (4T) as a combination pool and wetwell air space. The vent geometry was a single 24-in. vertical pipe. Particular emphasis was placed on pool response to a simulated loss-of-coolant accident including pool swell, wetwell pressurization, pool thermal response, and the phenomenon of chugging. Data correlations and phenomenological explanations of the observed data are included.