Two-dimensional (2D) Fourier transform ion cyclotron resonance (2D FT-ICR) allows one to obtain direct evidence for the occurrence of ion-mol. reactions. In 2D FT-ICR spectra, one observes resonances with 2 distinct frequency coordinates w1 and w2 that correspond to the mass-to-charge ratios of the ions that participate as reactants and products, resp. It is possible to monitor a manifold of distinct reactions in one single 2D spectrum. Spectral ranges of the order of several MHz can be covered by using frequency-swept ("chirped") radio-frequency pulses. [on SciFinder (R)]