Transformations of resins and asphaltenes of chemically different heavy oils during primary distillation have been studied using oils form the Usa (Komi Republic, Russia) and Zuunbayan (Mongolia) oil-fields as an example. The structural-group parameters of resins and asphaltene molecules of the crude oils and oil residues (fuel oils) obtained via atmospheric distillation involving the withdrawal of IBP-350A degrees C fractions have been determined using elemental analysis, molecular weight, and NMR data. Changes in the average structures of molecules in the primary distillation process of crude oils have been revealed, and it has been found that these changes depend on the chemical composition of the original oils.