The average structural characteristics of oily and resinous components of nine crude oils of various types from different Miocene horizons of the Sirakovo and Bradarac-Maljurevac oil fields (Serbia), as well as the mass-spectrometrically established hydrocarbon-group composition of these oils, are described. The most abundant components of crudes from both oil fields are mono- and bicyclanes bearing long alkyl substituents. It was found that there is no universal trend in the chemical type of the oils with the occurrence depth of the producing zones in these fields, that is, oils are enriched in aliphatic hydrocarbons with depth in the Sirakovo oil field and in naphthene hydrocarbons in the Bradarac field. These trends cannot be explained by microbial degradation processes or by the catagenetic alteration of petroleum composition in the Earth's interior and indicate a low level of thermal maturity of the source organic matter that gave rise to the hydrocarbon systems in question. A certain parallelism of variation in the structural parameters of hydrocarbonaceous and heteroorganic (resinous) components of the oils depending on natural factors is noted.