This paper discusses an FT-ICR mass spectrometry study of Russian and North Sea crude oils and their six distillation fractions (260-310, 310-360, 360-410, 410-460, 460-510, and 510-560 degrees C) by positive electrospray ionization (EST) mode. Only one major heteroatom class, N class (pyridine benzologues), was found for both crude oils and for every distillation fraction. The relative abundance of N class was found to be somewhat higher for North Sea oil than the Russian oil, containing higher series (DBE values) and carbon distributions. On the other hand, the second most abundant class found for crude oils, the NS class, was found to be more abundant for Russian than North Sea oil samples, in accordance with experimentally measured sulfur values for crude oils. Correspondingly, Russian oil contained a higher series and also carbon distributions NS compounds, in comparison to North Sea oil. The oxygen containing sulfur compounds (OS class) were present only at a low distillation temperature (< 410 degrees C) fractions. The results show that compounds present at trace amounts are more easily observed from distillation fractions than in crude oils.