Methodology is presented for the isolation and comparative determination of 16 alkyloyl and 2 heteroaroyl ecgonine methyl esters in illicit cocaine samples. These trace-level alkaloids, five of which are reported for the first time, were partially isolated from cocaine using alumina column chromatography and recrystallization and determined using capillary gas chromatography-mass spectrometry using selected ion monitoring. Acetoxy-, propionoyl-, isobutyroyl-, butyroyl-, 2-methylbutyroyl-, isovaleroyl-, valeroyl-, senecioyl-, trans-4-hexenoyl-, tigloyl-, hexanoyl-, 3'-furanoyl-, trans-2-hexenoyl-, 2'-furanoyl-, trans-3-heptenoyl-, cis-, trans-; trans-, cis-; and trans-, trans-2,4-hexadienoyl- ecgonine methyl esters were detected at levels less than 1 X 10-5 % (w/w relative to cocaine).