AbstractThe helically structured cholesteric and smectic C* phases have been induced by chiral guest molecules in liquid crystalline host systems exhibiting nematic as well as Scphases. The helical twisting power δ has been measured for those systems: i) In all cases values of δchol> δc*were obtained; ii) in the induced Sc* but not in the cholesteric phases there exists a threshold mole fraction x0Gof the chiral guest molecules below which no helical twist will be induced.—These experimental results are discussed in terms of the layer structure of the Scphase. By means of a simple structure model of the induces S*cphase the occurance of a threshold guest concentration can be understood. A relation between the ratio δchol/δc*and x0Ghas been derived from the model in good agreement with the experimental data.