A method is described for the accurate and precise determination of tributyltin (TBT) and dibutyltin (DBT) by species-specific isotope-dilution plasma-source mass spectrometry. Using gas chromatography (GC) for sample introduction and analyte separation, a performance comparison was made between sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) detection and quadrupole ICP-MS (qICP-MS) detection. Samples were extracted with acetic acid using open microwave digestion, derivatized with sodium tetraethylborate and extracted into isooctane. Mass bias correction was implemented based on the expected ratio of 120Sn/117Sn to that of the mean 120Sn/117Sn ratio calculated from the inorganic Sn peaks detected in all chromatograms. A more than 2-fold improvement in precision of calculated 120Sn/117Sn ratios was obtained for both TBT and DBT in standards using GC-SF-ICP-MS as compared to GC-qICP-MS. PACS-2 certified reference material marine sediment (NRCC, Ottawa, Canada) was used for method validation. Concentrations of 0.883 ± 0.013 and 1.126 ± 0.013 µg g−1 (mean and one standard deviation, n = 4) as tin were obtained for TBT and DBT, respectively, using GC-SF-ICP-MS detection, in agreement with the certified values of 0.98 ± 0.13 and 1.09 ± 0.15 µg g−1 (95% confidence interval), respectively. Concentrations of 0.883 ± 0.019 and 1.116 ± 0.014 µg g−1 (mean and one standard deviation, n = 4) as tin were obtained for TBT and DBT, respectively, using GC-qICP-MS detection. Slightly better precisions of 1.59–1.62% RSD for TBT and DBT in a test sediment were obtained using GC-SF-ICP-MS compared with 1.64–3.31% RSD obtained with GC-qICP-MS. Method detection limits (LODs, three times standard deviation) of 0.4 and 0.3 ng g−1 for TBT and DBT, respectively, were obtained using GC-SF-ICP-MS, based on processing a 0.5 g sample. As expected, these are superior to LODs of 0.9 and 1.0 ng g−1 obtained using GC-qICP-MS, arising from the three-fold enhancement in signal-to-background ratio obtained with the sector field machine.