Quantitative structure–activity relationships (QSARs) for metal bioconcentration factors (BCF) and median acute lethal water concentrations (LC50) were developed for two species of mollusks, Perna viridis and Mytilus edulis. These endpoints were related to four metal ion characteristics, the covalent index (χ2mr) (r represents the ion radius in Å), the softness index (σp), the hydrolysis constant (Koh) and the ionic index (Z2/r). The BCF and LC50 were significantly correlated to χ2mr. The coefficients of determination r2 for the relationships with other metal descriptors were much lower. Critical body residue (CBR) QSARs were derived by multiplying the χ2mr-based BCF and LC50 regressions. The CBRs were independent of the covalent index χ2mr, as BCF and LC50 scaled to χ2mr with slope that had opposite signs. Comparison of the estimated CBRs with independent empirical values confirmed the predicted trends, but substantial deviations were noted too.