This document specifies a method for the quantitative determination of saxitoxin (STX) and decarbamoyl saxitoxin (dc-STX) in mussels. It may also be applicable in other shellfish, for example scallops. The limit of determination of this method (signal/noise = 10) is 0,006 mg/kg for saxitoxin and 0,02 mg/kg for dc-saxitoxin in mussel meat. The method has been tested for saxitoxin at levels at 0,4 mg/kg and 0,5 mg/kg and for dc-saxitoxin at levels at 0,4 mg/kg and 1,6 mg/kg.