Abstract A voltammetric study has been made for the transfer of four 18-heteropolymolybdate anions including Dawson anions, i.e. [P 2Mo 18O 62] 6−, [As 2Mo 18O 62] 6−, [S 2Mo 18O 62] 4− and [P 2 Mo 18O 61] 4−, and of a related anion [S 2VMo 17O 62] 5− at the nitrobenzene(NB)—water(W) interface. With regard to the hexavalent and pentavalent anions, well-developed voltammetric waves due to their phase transfer could be obtained under certain conditions. The tetravalent anions, however, gave no wave in the polarizable potential range but showed an increase in the final current rise owing to their transfer from NB to W. From the voltammetric data, the standard ion transfer potential (Δ NB wφ j θ) and standard Gibbs transfer energy (Δ G tr,j θ,NB→W) have been determined (or estimated) for the heteropolyanions. It has been found that Δ NB Wφ j θ and Δ G tr,j θ,NB→W depend primarily on the ionic charge. This is in line with our previous result obtained for Keggin anions in 1991. A possible mechanism has also been proposed for the transfer of the hexavalent Dawson anions.