An automated method for trace level determination of 19 priority phenols in water samples has been developed using on-line liquid/solid extraction followed by liquid chromatography/mass spectrometry (LC/MS) with atmospheric pressure chemical ionization (APCI) and ion spray (ISP) interfaces in the negative ionization (NI) mode. Sixteen phenols were determined by LC/APCI-MS with high sensitivity. Three compounds were not detected by APCI-MS: phenol, 4-methylphenol, and 2,4-dimethylphenol could only be determined by ISP-MS using a porous graphitic carbon column with 100% of methanol as eluent. [M -H] -ion was the base peak using either LC/APCI-MS or LC/ISP-MS. Limits of detection ranging from 5 to 0.1 µg/L and 0.1-25 ng/L were found when 50-100 mL of river water was processed in full-scan and time-scheduled SIM modes, respectively. The proposed analytical method was validated by participating in various interlaboratory exercises distributed by Aquachek (WRC, Medmenham, U.K.), using groundwater samples containing phenols at levels ranging from 0.1 to 2.5 µg/L.