The effect of ion and radical compositions in BCl3/Cl-2 plasmas was assessed in this work with a focus on the formation of etch products in patterning hafnium aluminate, a potential high-k gate oxide material. The plasma composition became increasingly more complex as the percentage of boron trichloride was increased, which led to the formation of a significant amount of boron-containing species including B+, BCl+, BCl2, BCl3+, B2Cl3, and B2OCl3+ in the plasma. The BCl2+ ions were found to be the dominant species in BCl3 containing plasmas at most conditions; however, increasing the pressure or decreasing the power led to an increase in the formation of higher mass ions. Several compositions of Hf1-xAlxOy thin films ranging from pure HfO2 to pure Al2O3 were etched in BCl3/Cl-2 plasmas as functions of ion energy and plasma composition. The etch product distributions were measured and the dominant metal-containing etch products were HfClx and AlClx in a Cl-2 plasma and HfClx, HfBOCl4, and AlxCly in a BCl3 plasma, and their concentrations increased with increasing ion energy. Oxygen was detected removed in the form of ClO in Cl-2 and as trichloroboroxin ((BOCl)(3)) in BCl3. Both the etch rate and the etch product formation are enhanced in BCl3/Cl-2 plasmas, as compared to those in Cl-2 plasmas, due to the change in the composition and reactivity of the dominant ions and radicals.