A modified sample preparation method for oils for the determination of carbazoles is proposed, involving deasphalting of oil by settling a sample of oil in a 40-fold excess of n-hexane for one day and using non-modified silica gel instead of argentated. In this case, carbazoles are eluted in one fraction. Conditions for the identification and determination of carbazole, 9-methyl-, and 9-ethylcarbazole by gas chromatography with thermionic detection were selected, and the analytical characteristics of the method were established. The choice of 3-nitro-9-methylcarbazole as an internal standard for carbazole determination was justified, and its synthesis was performed. The presence and quantity of carbazoles in oils from different fields and in a synthetic oil sample were assessed. A comparison of carbazole determination results in natural oils using the external standard method, internal standard, and additions was conducted. It was shown that all methods yield the same results within the error limits. The carbazole content in natural oils for the studied samples was found to be in the range of 0.14–2.61 mg/kg. 9-Ethylcarbazole was detected in four oil samples.