The development of a highly sensitive system for the shipboard determination of dissolved iron at the sub-nM level is presented. The technique is based on a ฏow injection method coupled with luminol chemiluminescence detection. Dissolved Fe(IIIII) levels are determined after Fe(III) reduction using sulphite and in-line matrix elimination/preconcentration on an 8- hydroxyquinoline (8-quinolinol) chelating resin column. The detection limit (3s) is 40 pM when 1.5 ml of sample is loaded onto the column, and the relative standard deviation is 3.2% (nˆ5) for a 1.0 nM Fe sample. One analytical cycle can be completed in 3 min. The automated method proved reliable when employed on-board the RRS James Clark Ross during Autumn 1996, mapping dissolvable Fe(II‡III) levels along the Atlantic Meridional Transect from 508N to 508S. Data from vertical proฎles through the upper water column are presented