Azaspiracids (AZAs) are marine biotoxins that induce human illness following the consumption of contaminated shellfish. EU regulation stipulates that only raw shellfish are tested, yet shellfish are often cooked prior to consumption. Analysis of raw and heat-treated mussels (Mytilus edulis) naturally contaminated with AZAs revealed significant differences in toxin profiles due to heat induced chemical conversions. Consistent with previous studies high levels of AZA3, 3, and AZA6, 6, were detected in some samples that were otherwise below the limit of quantitation before heating. Relative to AZA1, 1, in heat treated mussels the average (n=40) levels of 3 (range 11−502%) and 6 (range 3−170%) were 62% and 31% respectively. AZA4, 4, (range <1–27%), AZA5, 5 (range 1–21%) and AZA8, 8 (range 1–27%) were each ~ 5%, while AZA7, 7, AZA9, 9 and AZA10, 10 (range <1–8%) were each under 1.5%. Levels of 5 and 10 (and AZA13, 13 and AZA15, 15) increased after heating leading to the identification of novel carboxylated AZA precursors in raw shellfish extracts which were shown by deuterium labeling to be precursors for 5, 10, 13 and 15.