Azaspiracids (AZAs) belong to a family of more than 50 polyether toxins originating from marine dinoflagellates such as Azadinium spinosum. All of the AZAs reported thus far contain a 21,22-dihydroxy group. Boric acid gel (BAG) can bind selectively to compounds containing vic-diols or α-hydroxycarboxylic acids via formation of reversible boronate complexes. Here we report use of BAG to selectively capture and release AZAs from extracts of blue mussels. Analysis of the extracts and BAG fractions by LC–MS showed that this procedure resulted in an excellent clean-up of the AZAs in the extract. Analysis by ELISA and LC–MS indicated that most AZA analogues were recovered in good yield by this procedure. The capacity of BAG for AZAs was at least 50 μg/g, making this procedure suitable for use in the early stages of preparative purification of AZAs. In addition to its potential for concentration of dilute samples, the extensive clean-up provided by BAG fractionation of AZAs in mussel samples almost eliminated matrix effects during subsequent LC–MS, and could be expected to reduce matrix effects during ELISA analysis. The method may therefore prove useful for quantitative analysis of AZAs as part of monitoring programs. Although LC-MS data showed that okadaic acid analogues also bound to BAG, this was much less efficient than for AZAs under the conditions used. The BAG methodology is potentially applicable to other important groups of natural toxins containing diols, including ciguatoxins, palytoxins, pectenotoxins,karlotoxins, tetrodotoxin, trichothecenes, and toxin glycosides.