Using octadecyl functional group (C18) bonded silica gel as sorbent and methanol or ethanol as eluent, the extractability of nickel, cobalt and manganese with diethyl-, di-n-propyl-, di-2-propyl-, di-n-butyl-, di-isobutyl-, di-2-butyl-, di-n-pentyl- and di-n-hexyldithiophosphates, (RO)2P(S)S-, was investigated in respect of the effects of pH, alkyl substituent group, reagent concentration and masking agent by flame atomic absorption spectrometry. The extractability increases with the chain length of the substituent groups in the reagents and is quantitative for alkyl groups larger than butyl at pH ca. 3. At a high concentration of di-n-pentyldithiophosphate, the metals are selectively extracted from coexisting cadmium, copper, lead and iron. As an analytical demonstration, di-n-pentyldithiophosphate was used for the determination of ultra-trace concentrations of nickel in two certified reference saline water samples (sea water and estuarine water) by graphite furnace atomic absorption spectrometry. Results were in good agreement with the certified values. The detection limit (3σ) of peak area measurement was 0.07 μg l-1 with an enrichment factor of 10 for nickel with 20-s sample loading at 8.7 ml min-1 and 20-μl eluate injection.