The extraction of Mo(VI) from its aqueous solutions into chloroform in the presence of 2,3-dihydroxy-naphthalene (DHN), and n-butyltriphenylphosphonium bromide (LBr), was investigated using 99-Ho tracer and the composition of the extracted compounds was studied. Mononuclear species were extracted at high DHN concentrations. The green extracts from strong acid aqueous solutions of H2SO4contained the compounds being formed from Mo, DHN and LBr in the ratio 1:2:1. Ion-pairs of the yellow anionic chelates of Mo(VI), Mo/DHN=l:l (monovalent) and 1:2, with phosphonium cations were extracted very efficiently from acid, neutral, and slightly basic media. If the DHN concentration was low, the extraction of oligonuclear (possibly complexed) species from strong acid media and oligomolybdates from slightly acid solutions was observed.