Progress in the development of advanced electronic and photonic devices strongly rely on the capability of etching multicomponent oxides such as Ca_xBa_(1-x)Nb₂O₆ (CBN), (Ba,Sr)TiO₃ (BST), and SrTiO₃ (STO) that exhibit ferroelectric and electro-optic properties of interest for these applications. An important issue in the development of plasma etching recipes for multicomponent oxides is that in contrast with simple, binary oxides such as SiO₂, ZrO₂ and HfO₂ the various atoms contained in the films are likely to interact differently with the various reactive species of the plasma. This makes investigations of the underlying physics and chemistry a very difficult task. In the present work, we propose a simple and effective way to examine the influence of surface chemistry on the plasma etching dynamics of multicomponent oxides. This method uses the energy dependence of the etch rate in combination with measurements of the total positive ion flux impinging onto the surface and relative positive ion composition of the plasma. Using pulsed-laser-deposited CBN and STO thin films as examples, it was found that the etching energy threshold, Eth, shifts towards values larger or smaller than the sputtering threshold depending on whether or not ion-assisted chemical etching is the dominant etching pathway. More specifically, displacement of Eth towards values larger than the sputtering threshold indicates an inhibiting surface chemistry while displacement towards lower energy is associated to an enhancing chemistry. For CBN films etched in an inductively coupled chlorine plasma, we measured Eth ~65 eV at 1 mTorr, ~240 eV at 10 mTorr, and ~400 eV at 15 mTorr. The threshold obtained in pure Ar plasma was similar to that achieved in Cl₂ at 1 mTorr, which suggests that CBN etching at low chlorine number densities is dominated by pure physical sputtering. This is consistent with TOF-SIMS measurements that showed comparable Ca, Ba, and Nb depth profiles for the samples etched in pure Ar and in 1 mTorr, Cl₂ plasmas. At 10 mTorr, the chlorine uptake was an order of magnitude higher than at 1 mTorr. In addition, we observed an important concentration of BaCl_x and NbCl_x, with a considerable amount of non-volatile BaCl₂and NbCl₂ closer to the topmost surface. Therefore, the higher etching threshold observed at 10 and 15 mTorr results from the formation of reaction products that are more difficult to etch than the bare CBN surface. A similar inhibiting chemistry was observed for STO films etched in a fluorinated plasma. Eth increased from ~50 eV in pure Ar to ~90 eV in a 30%SF₆-70%Ar plasma, with the desorption of SrF_x compounds being the rate-limiting step.