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The use of stable oxygen (δ18O) and carbon (δ13C) isotopic ratios of marine shell carbonate is a powerful tool for reconstructing past sea surface temperatures (SST) and estimating season of harvest for shells from coastal archaeological sites. While methods for sampling shells and analyzing the resulting data are established, less is known about the effects of anthropogenic activity on the geochemistry of the shells. Through an experimental study in which we heat carbonate powder from Mytilus californianus shells, we show that mussels cooked by boiling or steaming were unlikely to have their isotopic composition altered by the process. Shells heated over coals, however, show evidence of both visible and structural changes and in some cases are depleted in δ13C and/or δ18O. This indicates that archaeologists should use caution in interpreting past SST or site seasonality from burned shells and should instead test intact, unburned shells.