We have prepared oxides having various yttrium to aluminum ratios and exposed them to fluorine-based plasma. The etch rate of the aluminum oxide decreased abruptly with the addition of yttrium oxide and then slowly with further addition. The yttrium oxide had a more fluorinated surface than the aluminum oxide, indicating that the etch rate was not determined by the surface fluorination, contrary to the etching of Si-based materials. From the X-ray photoelectron spectroscopy (XPS) analysis of a multication YAlO(3) single crystal, a similar tendency was observed, showing a higher ratio of Y-F to Y-O bonding than the ratio of Al-F to Al-O. Angle resolved XPS and depth profiling analysis revealed the presence of a fluorinated layer of a few nanometer thick under a roughly 1 nm carboneous top layer. The etching behavior and surface chemical status of these oxides were discussed in terms of thermodynamic aspects of aluminum and yttrium fluoride.