The sensitive method DPASV with HMDE in the presence of DTPA and acetic buffer was used for determination of Tl(I) and Tl(III) in spiked seawater sample. The proposed method was based on the appropriate chemical conservation of Tl speciation. To determine the total amount of Tl, an addition of ascorbic acid to sub-sample was necessary, but to determine Tl(I) to the second sub-sample addition of DTPA was needed. Various factors influencing the stability of Tl(III) and electrochemical parameters were investigated. The quantification limit of 1.8 ng mL À1 Tl(I) was found for 8 min preconcentration. The procedure was validated using recovery study and HPLC ICP MS determinations.