Recycling of material and or energy in chemical plants imposes a feedback mechanism which is known to move the poles of the plant. In this paper the effect of recycling on the zero dynamics is considered, and it is shown that the feedback effect imposed by recycling also may move the zeros of control relevant transfer-functions. Sufficient conditions for the existence of non-minimum phase transmission zeros caused by the presence of recycling are derived. The conditions may be evaluated based on steady-state information about the individual units only, and are hence particularly well suited for incorporation in a process design environ ment. The derived results are illustrated through application to the design of a reactor-separator system.