The fine structure of isotopic peak clusters in mass spectra of reserpine and substance P are measured using Fourier transform ion cyclotron resonance mass spectrometry at a 7 T magnetic field. The resolved peaks in the fine structure consist of (13)C, (15)N, (17)O, (18)O, (2)H, (33)S, (34)S, and combinations of them. A recently introduced high-resolution ion cyclotron resonance cell (Nikolaev, E. N.; Boldin, I. A.; Jertz, R.; Baykut, G. J. Am. Soc. Mass Spectrom. 2011, 22, 1125-1133) is used in these experiments. The positions of the experimentally obtained fine structure peaks on the mass scale agree with the isotopic distribution simulations with ≤200 ppb error. Some deviation from the theoretical isotopic distribution is observed, less abundant peaks in the fine structure patterns are a little suppressed compared to the larger ones. We present a method for atomic composition determination using accurate mass data and fine isotopic structure of the mass spectrum. Our method combines the traditional atomic composition determination from accurate mass data by enumeration of all possible formulas within constraints defined a priori with the estimation of element coefficients from resolved isotopic structures. These estimated values allow one to narrow the search space for the composition and therefore to reduce the number of candidate formulas.