A study was made to illustrate how recycling in chemical conversion and separation systems can negatively influence the system and cause instabilities and increased sensitivity to disturbances. Two typical industrial processes were the examples, a facility to produce acetic acid and a facility to manufacture high pressure polyethylene. The observed phenomena were systematically studied and understood with the aid of a simple model. The model system considered consisted of an ideal stirred vessel reactor and a connected single stage flash with recycle. The dynamics of such a system depend decisively on the basic control strategy. Even for a simple case of an isothermal reaction of the first order as a result of recycling instabilities and ambiguities can occur. The dynamic instabilities remain even in non-reactive limiting cases and can lead in the case or a non-neglected transport holdup in the recycle to complex bi- and multi-rhythmic behavior. The other way around, with the help of these studies control strategies can be identified that always lead to clear and stable operating conditions.