The effects of eight transition metals were studied in a nitrifying system to investigate the relationship between the ionic characteristics of metals and their toxicity to nitrifiers. The cumulative oxygen consumption and the cumulative carbon dioxide production were monitored throughout each respirometric batch run to determine the toxicity of metals to nitrifiers. Several quantitative cationic-activity relationship (QCAR) models were developed on the basis of these different toxicity endpoints using quantum chemical descriptors. Descriptors were calculated with density functional theory (DFT) at the B3LYP/LANL2DZ level using the Gaussian 03W software. Additionally, the same descriptors were recalculated using another basis set, B3LYP/SDD, to test the impact of the basis set on prediction of toxicity. Of the calculated descriptors, mainly the gaseous phase descriptors explained significant variances in both toxicity endpoints. However, the energy of the polarized solute-solvent (EPSS) was the most important common descriptors in modelling labile toxicity. A combination of the aqueous phase energy of the highest occupied molecular orbital (EHOMO(aq)) and the maximum value for the energy of the lowest unoccupied molecular orbital of the most important metal species (ELUMOCmax) produced the best two-descriptor model for both pTO2 and pTCO2. The electron donor/acceptor ability of metals and the electron acceptor ability of metal species (ELUMOCmax) seemed to be important in explaining toxicity in aqueous media regardless of the measured endpoints for nitrifiers.