The occurrence of negative-ion species in a negative corona discharge in the Trichel regime in N₂/O₂ mixtures has been studied at pressures up to 1 bar. The problems of ion sampling from discharges containing chemically active species with respect to reproducibility and mass discrimination have been adequately solved. Ion sampling took place by probing the undisturbed supersonic expansion downstream from the extraction hole with a skimmer penetrating the surrounding shock wave. No electric fields were applied between the extraction hole and the skimmer. The experiments were carried out in gas containing less than 5 volume parts per million (VPM) water and showed that the negative ions formed in the discharge chamber will cluster with water molecules. The complicated negative-ion mass spectra occurring could only be identified assuming that in the discharge chamber clustering with H₂O occurs with clustering numbers up to three and that clustering in the supersonic expansion with air molecules (mainly N₂) occurs with clustering numbers up to seven.