A method for the simultaneous determination of methylated, ethylated and butylated tin compounds in landfill leachates has been developed in this work. The assessment of the organotin compound composition has been achieved by the development of a specific GC-ICP-MS protocol adapted to these complex matrices. The analytical procedure consists in three major steps which have been carefully optimized, taking into account the variety of alkyltin compounds and the high organic content of the leachate matrix: nitric acid digestion under microwave, derivatization using sodium tetrapropylborate and chromatographic separation. Different quantification approaches are proposed for the determination of the alkyltin species in the leachates. In this way, isotope dilution analyses in the species-specific and species-unspecific spiking modes have been found to provide results in agreement with external and internal calibration approaches. A single analysis with an addition of three isotopically enriched butyltin species is found to be suitable for the routine quantitative and semi-quantitative determination of all occurring alkyltin species in landfill leachates. The different qualitative and quantitative GC-ICP-MS complementary approaches developed in this work allow the full organotin composition assessment of landfill leachates.