Experiments are performed with a single‐stage magnetic fluid rotary feed‐through seal to determine the influence of shaft rotation, seal fluid inventory, seal clearance, magnetic field intensity, and magnetic field distribution on the maximum seal capacity or burst pressure. For small magnetic fluid inventories, the seal exhibits very low static capacity, a maximum capacity near 20 Hz, and a slowly decreasing capacity for higher shaft speeds. For larger inventories, the effect of motion is small. Under a constant inventory condition, the larger clearance seals are less sensitive to shaft motion than the smaller clearance seals. As the seal clearance is increased, inevitable fluid loss due to rotation becomes more pronounced and the seal becomes weaker. For decreased magnetic field intensity in the gap, the seal capacity exhibits increased sensitivity to shaft speed. Several hypotheses are proposed to explain the observed behavior. Certain design‐related implications are discussed.