In this study, we examined the masking effects of 16 water-miscible reagents, on the extraction of Mo, Re, Ru, and Pd. These reagents included soft N- and/or S- donor atoms in their structures and they function as multidentate ligands. The extractants, methylimino-N,N-dioctylacetamide (MIDOA) and N,N,N;N',N",N"-hexaoctyl-nitrilotriacetamide (NTAamide(C8)), show significantly high distribution ratios for these metals(D(M)), were employed in this study. Masking effects were observed as a decrease in D(M) with an increase of masking agent concentration in these extraction systems. The results showed that D(Pd) can be considerably decreased by the addition of thiourea, cysteine, cysteamine, trisaminoethylamine, and diethylenetriamine, and D(Ru) is decreased by the addition of thiourea, trisaminoethylamine, and diethylenetriamine. Pd and Ru can be masked by similar reagents including N- or S- donor atoms, which suppressed the extraction into the organic phase. In contrast, D(Mo) was only slightly masked by the above-mentioned reagents. The masking of Mo was achieved using methylimino-N,N'-diethylacetamide, hexaethyl-nitrilotriacetamide, iminodiacetic acid, and iminoclimethylphosphonic acid, which have a central N(CH2C(P)=O)(2) framework that is important for this purpose. A masking agent for Re was not found in this study.