Abstract: A use composition monitor determines the concentration of peracid and/or peroxide in a use composition using a kinetic assay procedure. A sample mixture containing a sample of the use composition, a diluent and at least one reagent is prepared and analyzed using, for example, an optical detector. Response data obtained by the detector is indicative of the optical absorbance of the sample mixture as a function of time. A processor analyzes the response data to determine a corresponding best fit linear relationship. The initial absorbance of the sample mixture is indicative of the concentration of peracid in the use composition, while the slope of the best fit equation is indicative of the concentration of peroxide in the use composition.