Dissolved barium and molybdenum incorporation in the calcite shell was investigated in the Great Scallop Pecten maximus. Sixty six individuals were exposed for 16 days to two successive dissolved Ba and Mo concentrations accurately differentiated by two different isotopic enrichments (⁹⁷Mo, ⁹⁵Mo; ¹³⁵Ba, ¹³⁷Ba). Soft tissue and shell isotopic composition were determined respectively by quantitative ICP-MS (Inductively Coupled Plasma Mass Spectrometer) and laser ablation--ICP-MS. Results from Ba enrichment indicate the direct incorporation of dissolved Ba into the shell in proportion to the levels in the water in which they grew with a 6-8 day delay. The low spike contributions and the low partition coefficient (D(Mo) = 0.0049 ± 0.0013), show that neither the soft tissue nor the shell were significantly sensitive to Mo enrichment. These results eliminate direct Mo shell enrichment by the dissolved phase, and favour a trophic uptake that will be investigated using the successive isotopic enrichment approach developed in this study.