The room‐temperature ion‐assisted etching of poly‐Si with molecular chlorine and 1 keV Ar+ions has been studied for a wide range of the neutral flux/ion flux ratio.Insitumeasurements of the etch rate made using quartz crystal microbalance methods are combined with modulated beam mass spectrometric studies of the etch products and the unreacted molecular chlorine reflected from the Si surface. The reaction probability for the incident chlorine is determined in two independent ways and good agreement is obtained. The products are determined to be primarily SiCl2and SiCl4with significant amounts of sputtered SiCl at low flux ratios.