Abstract: Relative ion abundances in ion cyclotron resonance mass spectrometry are determined utilizing wavelet transforms to isolate the intensity of a particular ion frequency as a function of position or time within the transient ion cyclotron resonance signal. The wavelet transform intensity corresponding to the frequency of each ion species as a function of time can be determined, an exponential decay curve fitted to such data, and the decay curves extrapolated back in time to the end of the excitation phase to determine accurate values for the relative abundances of the various ions in a sample. By determining the abundances of ions at a point in time at or near the end of excitation, the effects of different rates of decay of the intensity of the signal from different ions species can be reduced, and more accurate ion abundance measurements obtained.