This repository contains (i) all the data included in the study "Ligand exchange provides new insight into the role of humic substances in the marine iron cycle" in seven XSLX files, and (ii) twelve scripts and eight CSV files used to model the data with Dynafit software (http://www.biokin.com/dynafit/). A readme file is included to guide the calculations. The data sets originated when iron-saturated complexes of humic substances (fulvic acid and humic acid from the International Humic Substances Society) in seawater were subjected to iron ligand exchange after addition of an iron-free competing ligand, in most cases desferrioxamine B. In four cases EDTA and protoporphyrin IX were used as competing ligands to understand the effect of the chemical nature of the competing ligand. Additionally, reversibility was also tested. The concentration of the Fe-humic complexes was monitored by catalytic cathodic stripping voltammetry (10.4319/lo.2009.54.2.0610), a method not sensitive to the other iron species and free iron present.