Due to the operation at background pressures between 10 -40 mbar and high reduced electric field strengths of up to 120 Td, the ion-molecule reactions in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) differ from those in classical ambient pressure IMS. In the positive ion polarity mode, the reactant ions H + (H 2 O) n , O 2 + (H 2 O) n and NO + (H 2 O) n are observed in the HiKE-IMS. The relative abundances of these reactant ion species significantly depend on the reduced electric field strength in the reaction region, the operating pressure, and water concentration in the reaction region. In this work, the formation of negative reactant ions in HiKE-IMS is investigated in detail. Based on kinetic and thermodynamic data from literature, the processes resulting in the formation of negative reactant ions are kinetically modeled. To verify the model, we present measurements of the negative reactant ion population in the HiKE-IMS and its dependence on the reduced electric field strength as well as the water and carbon dioxide concentration in the reaction region. The ion species underlying individual peaks in the ion mobility spectrum are identified by coupling the HiKE-IMS to a time-of-flight mass spectrometer (TOF-MS) using a simple gated interface that enables the transfer of selected peaks of the ion mobility spectrum into the TOF-MS. Both, the theoretical model as well as the experimental data suggest the predominant generation of the oxygen based ions O -, OH -, O 2 -, and O 3in purified air containing 70 ppm v of water and 30 ppm v of carbon dioxide. Additionally, small amounts of NO 2and CO 3are observed. Their relative abundances highly depend on the reduced electric field strength as well as the water and carbon dioxide concentration. An increase of the water concentration in the reaction region results in the generation of OHions, whereas increasing the carbon dioxide concentration favors the generation of CO 3ions, as expected.of ions in the reaction region might lead to an enhancement of the linear range and a decrease in chemical cross sensitivities. Second, at elevated reduced electric field strengths, all charge bound cluster formation equilibria are shifted towards smaller sizes, enabling potentially new ionization pathways to ionize e.g. low proton or electron affine substances not detectable when using ambient pressure chemical ionization 2 . Other major benefits are orthogonal ion separation using the fielddependent ion mobility (alpha-function), known from field asymmetric ion mobility spectrometers (FAIMS) and differential ion mobility spectrometers (DMS) 5,6 , and ion collision induced fragmentation for improved compound identification 7,8 . However, due to the operation at decreased pressures and high reduced electric field strengths, the ion-molecule chemistry in the HiKE-IMS generally differs from that in classical ambient pressure IMS. In order to predict the ionization pathways of specific analyte molecules, a detailed knowledge about the dominant reactant ion species generated...