The fundamental rotational transition J = 1←0 of the anion OH(-) has been measured by cooling mass-selected OH(-) ions to 10 K in a 22-pole ion trap and applying a novel rotational-rovibrational two-photon scheme. A transition frequency of (1 123 101.0410 ± 0.0014) MHz was obtained with so far unprecedented accuracy. The general application of the presented action-spectroscopy scheme to other anions and cations is discussed.