The adsorption of the ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][BF4] on ZnO (0001) and ZnO (10-10) has been studied using synchrotron-based soft X-ray photoelectron spectroscopy. Results indicate that [C8C1Im][BF4] is deposited intact on the ZnO (0001) surface, however, there is some dissociation of [BF4]- anions resulting in boron atoms attaching to the oxygen atoms in the ZnO surface and forming B2O3. In contrast, the deposition of [C8C1Im][BF4] on the ZnO (10-10) surface at -150 °C results in the appearance of more chemical environments in the XPS spectra. We propose that the high temperature of the IL evaporator causes some conversion of [C8C1Im][BF4] to a carbene-borane adduct, where BF3 attaches to the carbon atom between the two nitrogen atoms in the imidazolium ring. This results in deposition of both the IL and adduct onto the ZnO surface. The adsorption and desorption of the analogous IL 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1Im][BF4] was investigated on polar ZnO (0001) using synchrotron-based soft X-ray photoelectron spectroscopy. Results indicate that [C4C1Im][BF4] is deposited largely intact at -150 °C and forms islands when heated to room temperature. When heated to over 80 °C it begins to react with the ZnO surface and decompose. This is a much lower temperature than the long-term thermal stability of the pure IL, quoted in literature as ~400 °C, and of IL on powdered ZnO, quoted in literature as ~300 °C. This indicates that the ZnO surface may catalyse the thermal decomposition of [C4C1Im][BF4] at lower temperatures. This is likely to have a negative impact on potential use of ILs in photovoltaic ZnO-based applications, where operating temperatures can routinely reach 80 °C.