Dialkyldithiophosphates, (RO)2P(S)S-, were used as the complexing agent for flow injection on-line sorbent extraction for the determination of zinc by flame atomic absorption spectrometry. The extractability with diethyl-, di-n-propyl-, di-2-propyl-, di-n-butyl-, di-isobutyl-, di-sec.-butyl-, di-n-pentyl- and di-n-hexyldithiophosphates was compared in respect of the effects of pH, alkyl group, masking agent and reagent concentration. Di-sec.-butyldithiophosphate (0.5%, m/v) at pH 3 with 0.1 M citrate as a masking agent for iron, was used for the determination of zinc in a number of certified biological and environmental reference materials from the Community Bureau of Reference (BCR). All analytical results were in good agreement with the certified values. The detection limit (3σ) of peak height measurement, using a flow spoiler in the spray chamber, was 0.5 μg l-1 with an enhancement factor of 35 for a 20-s sample loading at 8.7 ml min-1.