The aims of the study were to determine the best method for extracting oil absorbed on coalescing filters at compressed natural gas (CNG) refueling stations and to compare the mass spectrometer (MS) and flame ionization detector (FID) for the quantification of the oil recovered in the extracts. Dichloromethane and heptane as solvents gave slightly higher recovery yields than pentane. The preferred extraction method with regard to time and solvent consumption consisted of an ultrasonic extraction, followed by removal of the remaining solvent under a stream of nitrogen. The FID and MS were found to be equally suitable for quantifying oil carryover, if the sample only contained the target oil when the instruments of analysis have been properly calibrated. If the sample is contaminated by compounds other than the target oil, MS and FID will provide different valuable information: MS may give information on the structure of the contaminants, while FID will give a more reliable quantification without proper calibration. The work discusses issues with the reusability of the filters and how to handle the memory effects.