The foam-breaking characteristics of rotating-disk mechanical foam-breakers (MFRDs) fitted to stirred-tank reactors (STRs) containing various foaming liquids were evaluated, The critical disk rotational speed, N-c, required for foam-breaking and the liquid hold-up, phi (L), in ascending foam reflected, respectively, the foam-breaking behaviour of MFRDs and the foaming behaviour of STRs, Empirical equations for the prediction of N-c and phi (L), which can be applied independently of the type, concentration and temperature of foaming liquid, were obtained, The foam-breaking power, P-kc, Of MFRDs was also clarified in relation to the level of phiL which is related to the difficulty or ease of mechanical foam-breaking.