Abstract Azaspiracids (AZAs) are a group of polyether toxins that cause food poisoning in humans. These toxins, produced by marine dinoflagellates, accumulate in filter-feeding shellfish, especially mussels. Sensitive liquid chromatography-electrospray ionisation mass spectrometry (LC-ESI-MS n ) methods have been developed for the determination of the major AZAs and their hydroxyl analogues. These methods, utilising both chromatographic and mass resolution, were applied for the determination of 10 AZAs in mussels ( Mytilus edulis). An optimised isocratic reversed phase method (3 μm Luna-2 C 18 column) separated 10 azaspiracids using acetonitrile/water (46:54, v/v) containing 0.05% trifluoroacetic acid (TFA) and 0.004% ammonium acetate in 55 min. Analyte determination using MS 3 involved trapping and fragmentation of the [M+H] + and [M+H−H 2O] + ions with detection of the [M+H−2H 2O] + ion for each AZA. Linear calibrations were obtained for AZA1, using spiked shellfish extracts, in the range 0.05–1.00 μg/ml ( r 2=0.997) with a detection limit of 5 pg (signal:noise=3). The major fragmentation pathways in hydroxylated azaspiracids were elucidated using hydrogen/deuterium (H/D) exchange experiments. An LC-MS 3 method was developed using unique parent ions and product ions, [M+H−H 2O−C 9H 10O 2R 1R 3] +, that involved fragmentation of the A-ring. This facilitated the discrimination between 10 azapiracids, AZA1–10. Thus, this rapid LC-MS 3 method did not require complete chromatographic resolution and the run-time of 7 min had detection limits better than 20 pg for each toxin.