A microwave-assisted extraction (MAE) method for the determination of Irgarol 1051 (2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine) and its main degradation product M1 (2-methylthio-4-tert-butylamino-s-triazine) in marine sediments by gas chromatography-mass spectrometry (GC-MS) was developed. The key parameters of MAE procedure, including the amount of the sediment, the volume of the extraction solvent, the duration and the temperature of the extraction procedure were optimized. The extraction procedure was followed by solid-phase extraction (SPE) on reverse phase C18 cartridges. The isolation of the target compounds from the matrix was found to be efficient when 3 g of marine sediment were extracted with 30 ml of water for 10 min at 115 degrees C. Final determination was accomplished by GC-MS. Quantification was performed with matrix-matched calibration using atrazine-d5 as internal standard. Mean recoveries higher than 85.4% were obtained for both compounds at three fortification levels with relative standard deviations (R.S.D.) < or = 14%. The limits of detection (LOD) of the developed method were 0.9 and 1.7 ng g(-1) dry weight for M1 and Irgarol 1051, respectively.