The negative and positive ion mass spectra from radio frequency glow discharges in boron trichloride and 90% argon‐10% boron trichloride mixtures have been investigated by amplitude modulation of the discharge excitation. The negative ion spectrum from pure boron trichloride was found to extend beyond 200 amu under most conditions even though positive ions above 120 amu were not detected under any conditions. Surprisingly, only trace signals of BCl−3were found from the discharges, while the signals from Cl−, BCl−4, B2Cl−4, and B2Cl−5were much larger. We were unable to detect either positive or negative ions above 120 amu from 10% boron trichloride discharges.