Several complexing agents with a soft donor incomporated into their structures are examined for the masking effects of Mo and Re through solvent extraction. The available extractants are N,N,M,N',N '',N ''-hexaoctylnitrilotriacetamide and N,N,N',N'-tetraoctylmethyliminodiacetamide, whose extractants have high extractabilities. Mo and Re dissolved in HNO3 are clearly suppressed by masking agents with N-donor ligands, such as ethylenediamine, diethylenetriamine, and 1,3-diaminopropane, for extraction into the organic phase. Ligands that include a sulfur donor, e.g., thiourea, were also tested, but exhibited a weaker masking effect.