In the recent years MARIN has developed a particular testing approach for the measurement of the reaction forces of free-surface anti-roll tanks. The objective of the present paper is to show how this approach, which has been followed throughout the different projects where this approach was applied, has led toobtain an accurate picture of the behaviour of free-surfaceARTs. In the first section, the reader is introduced to the working principle of free-surface ARTs and the usual model testing techniques applied to characterise their behaviour. In a further section, the approach followed by MARIN is described with detail, and the merits of such approach are highlighted. Finally, some of the test results are discussed.