The characteristics of four mechanical foam-breakers (a six-blade turbine (F-B), a six-blade vaned disk (V-D), a two-blade paddle (T-P) and a conical rotor (C-R) having six-blades fixed internally) with rotating parts fitted to a bubble column (BC) treating various foaming liquids were examined. In the BC with different foam-breakers, the average liquid hold-up in foam, phi(L), was both a typical parameter reflecting the foaming intensity of the BC, and a parameter related to the difficulty or ease of foam-breaking. The value of power consumption, P-kc, for foam-breaking in the C-R was the largest amongst the foam-breakers. Furthermore, it was demonstrated that in the four foam-breakers with rotating parts a considerable amount of very small liquid droplets was entrained with the exhaust air.