Raw and treated wastewaters from a municipal wastewater treatment plant were analyzed for presence of nonsteroidal anti-inflammatory drugs (ibuprofen, naproxen, ketoprofen, tolmentin and fenoprofen) by the solid phase extn. with liq. chromatog.-tandem mass spectrometry with a multiple addn. of std. samples. Solid matrix effect, extn. efficiency and process efficiency were detn. The std. sample addn. was recommended for reliable and results.