The invention relates to the production of water cluster ions (“hydronium clusters”) at atmospheric pressure for the chemical ionization of analyte molecules. It is proposed that a corona discharge at the Taylor cone of an aqueous liquid, preferably pure or slightly acidified pure water, is used instead of corona discharges on metal tips, which have been the usual method up to now. The hydronium clusters of the form [H(H2O)n]+ can be produced in a discharge chamber, which is separate from the ionization chamber, and introduced into the ionization chamber through a capillary. In the ionization chamber, the hydronium clusters can be heated and reduced in size by means of electrical acceleration and gas collisions, and thus made more reactive in order that analyte molecules of low proton affinity can also be ionized.