Selective reactive ion etching of Si over Si1-xGex and Si1-xGex over Si has been demonstrated by using a modulation-frequency plasma-etch technique which employs CHF3 and H-2 as the etch precursor gases. The selective etch crossover region appears at a modulation frequency of 2-3 Hz for a duty cycle of 50%. It is suggested that the etch selectivity phenomenon arises from the relative ion-assisted and purely chemical compounds of the radio frequency plasma and decaying plasma afterglow.