The objective of this study was to determine the effect of thermal treatment (frying) of frozen seafood on the content of minerals and heavy metals. An analysis was conducted to compare concentration of sodium, magnesium, potassium, calcium, copper, iron, manganese, zinc, as well as lead, cadmium and mercury in the analysed tissues of invertebrates (shellfish – blue mussel, Japanese squid, white shrimp and octopus). Thermal processing (in this case, frying) applied to the frozen invertebrates was found to affect significantly the content of macro- and microelements as well as the content of heavy metals, except for potassium and manganese content in mussels and shrimps, respectively. Frozen and thermally treated (frying) mussels are characterised by significantly higher concentrations of the analysed minerals (except Na+, Ca+2, Cu+2), as well as higher concentrations of cadmium and mercury. In turn, shrimps and squids are invertebrates with lower content of both macro- and microelements.