We optimised the simultaneous analysis of four analytes, namely three anti-influenza drugs (oseltamivir (OS), zanamivir (ZAN) and amantadine (AMN)) and one metabolite of OS (oseltamivir carboxylate (OC)), in sewage treatment plant (STP) influent and effluent by verifying the types and conditions of solid phase extraction (SPE) appropriate for LC-MS/MS analysis. In summary, these target analytes were extracted from aqueous samples (30-50 mL) by using strong cation-exchange SPE cartridges (500 mg adsorbent) under acidic conditions (pH 3-4). After washing of the cartridges with acidified water (pH 3.0, 3 mL) and methanol (3 mL), the analytes were eluted with a mixed solvent (2 mL) of 10% (v/v) triethylamine in a 1:1 (v/v) mixture of acetone and water. Application of this technique to the target compounds should yield a comprehensive understanding of the occurrence and fate of anti-influenza drugs in the water environment.