Quadrupole mass spectrometry (QMS) has been used as aninsitudiagnostic for GaAs etching with a Cl2/Ar plasma generated by an electron cyclotron resonance source. Partial pressure from reactive species and volatile etch products down to 10−10Torr can be detected. AsCl+x(x=1–3) products have been detected from 6×6 mm2GaAs samples. The145AsCl+2signal is generally the strongest. For an increase of microwave power from 50 to 400 W, the GaAs etch rate increased from 151 to 263 nm/min and the145AsCl+2partial pressure increased from 0.6×10−8to 2.8×10−8Torr. The effects of changing source distance, rf power, and pressure were also studied and the changes in etch rate can be correlated to the mass spectrometric intensities of AsCl+x. QMS has been used to monitor the etching of surface oxide on GaAs for different dc biases on the sample. It is found that the oxide etch time is a function of ion energy. For a self‐induced dc bias of −100 V, the145AsCl+2signal remained at 1.4×10−9Torr for the first 15 s of etching. After the oxide was removed the signal increased to 5.5×10−9Torr. O2and N2were added to determine the effects of residual gases in the plasma system. Adding 2.5% O2in the Cl2/Ar discharge caused significant reduction in the etch rate from 171 to 43 nm/min and the145AsCl+2intensity was reduced from 8.0×10−9Torr down to the noise level. For complete substitution of N2for Ar the etch rate only dropped by 1/3 and the partial pressure of the etch products showed a similar decrease. O2was added during an etch to determine the sensitivity of the mass spectrometer to changes in plasma conditions. The decrease in the145AsCl+2signal intensity follows the increase in the16O+2signal intensity with less than 2 s delay.