The oscillatory system of a rotational viscometer has been investigated. The oscillation equations were checked with experimental bob deflection‐time curves obtained for Newtonian liquids.To be able to rapidly obtain correct values when taking flow curves the oscillatory system should be critically damped. Since this condition cannot be easily obtained in practice, the system should be slightly overdamped, meaning that the inertia (I) should be equal or smaller than &pgr;&mgr;2/720KS2. It is shown that due to the higher viscosity of thixotropic materials at lower rates of shear, the flow curves at these rates of shear give an indication of rather too low than too high shearing stresses.