Free bromine and bromamine analysis by the diethyl-p. phenyl-lene-diamine (DPD) colorimetric method requires some care when chloride and bromide are present at seawater concentration. If no particular problem is encountered in mono and dibromamine determination, on the contrary the analysis of either free bromine or that of a free bromine and tribromamine mixture can induce errors of at least twenty per cent in the total residual oxidant concentration due to halogen loss. In order to prevent that phenomenon, iodide addition into the phosphate buffer and DPD mixture is necessary before the sample introduction. In these conditions the results are in good agreement with those of the phenol red method.