Abstract: Fine structures of isotopic peak clusters of substances are determined using ultrahigh resolution mass spectrometry, e.g, FT-ICR mass spectrometry. Resolved individual peaks in the fine structure of the non-monoisotopic peak clusters of organic substances usually contain the additional elemental isotopes 13C, 15N, 17O, 18O, 2H, 33S, 34S, and combinations thereof. In each of a series of experiments, one of the non-monoisotopic peak clusters is isolated and the corresponding fine structure spectrum acquired. Abundances of the resolved fine structure peaks and their positions on the mass scale are recorded and, after measuring some or all of the isotopic peaks, the atomic composition of the measured substance is calculated. By excluding the monoisotopic peak and isolating only one isotopic peak cluster at a time, the number of ions in the FT-ICR cell is kept low, which avoids resolving power losses due to space charge effects and ion-ion interaction phenomena.