The stability of circulation patterns in stirred tanks was examined using tuft flow visualization. Tufts were placed throughout the tank and their direction was recorded continuously for 20 min. A direction coefficient, which allowed comparison of the stability between various tank geometries, was defined and measured. Using a two-level factorial design, the influence of four different geometric variables (impeller type, impeller diameter, off-bottom clearance and number of baffles) was examined. The impeller diameter, the number of baffles, and the interactions of the type of impeller with the number of baffles, and the type of impeller with off-bottom clearance affected the directional stability of the flow. This study has confirmed our understanding of time-varying phenomena in stirred tanks, and clarified the more complex interactions between elements of the geometry.