A method was developed for the quantification of alternariol and alternariol monomethyl ether on tangerines with and without symptoms of Alternaria brown spot disease. The method employs solid-phase extraction for cleanup, followed by high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS) for detection. This method was validated on flavedo (exocarp or epicarp, exterior yellow peel) and on albedo tissue (mesocarp, interior white peel). An excellent linearity over a range of 0.50-20.0 mg/kg was achieved, with r2 >or= 0.997. The limits of detection (LOD) and quantification (LOQ) were fewer than 0.13 and 0.50 microg/kg, respectively. The relative standard deviations (RSDs) were <or=14.4% during the validation. The levels of these mycotoxins on flavedo of fruits with symptoms vary from 2.54 +/- 0.24 to 17.40 +/- 1.05 mug/kg. Surprisingly, neither alternariol nor altenariol monomethyl ether was detected on albedo tissues, suggesting that flavedo works as a barrier for such substances.