The effect of Zeff on the runaway generation process by close collisions has been studied experimentally in the TEXTOR-94 tokamak in ohmic low density discharges. It is shown that the effective avalanching time increases with increasing Zeff. This opens the possibility of controlling the runaway production in tokamaks by active impurity injection. Furthermore, the energy of the runaway electrons emitting the observed synchrotron radiation could be determined to be Wr ≈ 20 MeV. However, for ITER-like disruptions the runaway production will probably be reduced but not prevented by a Zeff increase.