Abstract: The present invention relates to a method for determining a concentration of a scale inhibitor in a salt water comprising an analysis with a calcium/magnesium ionselective electrode of a dialyzed first sample of the salt water, and a dialyzed second sample of the salt water which was supplemented with a known concentration of the scale inhibitor. The invention further relates to a method for inhibiting incrustation in a plant which contains a salt water comprising the steps of adding a scale inhibitor to the salt water at a desired concentration, determining the actual concentration of the scale inhibitor in the salt water as above, and adding further scale inhibitor to the salt water to adjust the desired concentration.