Abstract: The zone penetration technique in a flow injection system is used for on-line dilution determination by spectrophotometry. The influence on dilution is investigated, and includes flow rate, reactor length and diameter, as well as sample volume and penetration zone length. In addition, this on-line dilution method is used to determine if magnesium is present in water samples. The results of the experiment show that this method is easy to use and convenient to handle. It is especially suitable for rapid determination of high concentration samples by spectrophotometry.