The phase transfer mechanism of 18-molybdophosphate anion at the water/nitrobenzeneinterfaca has been investigated by chronopotentiometry with cyclic linear current-scanning (CLC)and cyclic voltammetry (CV). The transfer species is 18-molybdophosphtae anion with a chargenumber of-4, H_2[P_2Mo_(18)O_(62)]~(4-). The transfer process is controlled by diffusion at a slow polarizationrate and considerably influenced by pH of the aqueous phase. The stable forms and pH range ofthe heteropoly anion in the aqueous solution can be directly confirmed through voltammetric behavior.The theoretical analysis of the relationship between the transfer potential and solution pH is identicalto the experimental results. The linear concentration relationship with the transfer peak current issuggested to be used in the determination of heteropoly acids(salts).