Abstract Specifies a procedure for water, readily applicable to field testing; it is based on measurement of the colour intensity by visual comparison of the colour with a scale of standards which is regularly calibrated. Sea water and waters containing bromides and iodides comprise a group for which special procedures are required. The procedure is applicable to concentrations, in terms of chlorine, from 0,000 4 to 0,07 mmol/l total chlorine and at higher concentrations by dilution of samples. Interferences of the procedure are noted.