On the basis of NMR spectroscopy, a method is proposed for calculating the average molecular weight, average number of double bonds and average degree of their substitution in molecules of unsaturated petroleum hydrocarbons. For the case of analysing specimens of crude oils from six fields of different oil- and gas-bearing regions it is shown that unsaturated hydrocarbons are based on monoenes with an average degree of substitution of the double bond of 2.4-3-2, and the average number of C atoms is 25-40.