Free surface anti-roll tanks are commonly used to reduce the rolling of ships. These devices have been studied quite extensively in the past, both analytically and experimentally. Most of these studies focused on the tank response to roll motions; in some work the effects of roll induced sway were addressed by varying the vertical position of the tank with respect to the rotation point. To obtain a more definite answer on the effects of sway in the response of anti-roll tanks, the present work investigated their response under arbitrary combinations of roll and sway motions. Using a 6 degrees of freedom hexapod table, harmonic and irregular forced motion tests were performed in roll, heave and sway, both independently and combined.