Changes that occur at the molecular level in the composition of nitrogen compounds of a petroleum sample from the Yuzno-Yagunskoe field during ozonation have been studied by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry in soft ionization modes. It has been found that N1Ox-containing products accumulate to form new classes of compounds, namely, N1O2, N1O3, N1O4, N1O5, N1O6, and N1O7, while N-1 classes undergo degradation. Ozonation of 50% of the feedstock leads to a similar to 30% decrease in the number of identified empirical formulas of pyridine bases (N-1 class).