Hydrocyclones are state of the art technology for treating produced water offshore. In the acceleration field inside the hydrocyclone oil migrates to the cyclone centre as water is forced to the wall. Separation is depending on the characteristic two-fold vortex flow structure featuring hydrocyclones. Deoiling hydrocyclones do not always meet water quality requirements, however. This paper, describes experimental investigations and hydrocyclone efficiency during transient flow rates. A laboratory test rig exposed a hydrocyclone to severe transient flow rates. Efficiency disturbances may be detected by increased oil-in-underflow, which again is associated with vortex disturbances and plausible vortex breakdown. Experiments led to no such dramatic disturbances. However, separation effects traceable to a limited influence on the flow split were realized. This investigation emphasizes the importance of adequate hydrocyclone operational control. Based on experimental results it is concluded that hydrocyclone efficiency is unaffected by transient flow rates provided the hydrocyclone performance criteria are fulfilled. (c) 2007 Elsevier Ltd. All rights reserved.