The influence of the plasma positive ion composition on the ion-assisted chemical etch yield of SrTiO₃ (STO) and Ca_xBa_1-xNb₂O₆ (CBN) thin films was investigated. In plasmas produced in molecular gases, the ions are known to fragment into their constituent particles upon impact on the material surface. The kinetic energy acquired by the ions in the plasma sheath is then shared between the various dissociation products. Therefore, the net etch yield can be expressed as the sum of the etch yields associated with each fragment. This effect was quantified by measuring the relative density of all ion species impinging onto the surface using plasma sampling mass spectrometry. It was found that the effective ion mass increased from 40 to 90 AMU for STO samples etched in Ar/SF₆ plasmas and from 44 to 58 AMU for CBN samples etched in Cl₂ plasmas. For STO, the etch yield decreased as the concentration fraction of molecular ions increased. This trend was in excellent quantitative agreement with the result of a simple rate model accounting for the dissociation of molecular ions upon impact on the surface. A similar behavior was observed for CBN films etched in Cl₂. However, in this case, the increase of the effective ion mass cannot fully explain the behavior of the etch yield with varying operating parameters (pressure and Cl₂ concentration in Ar/Cl₂); variation of the Cl coverage also needs to be taken into account.