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 used to determine the concentrations of peracid and/or peroxide in the use composition based upon an evaluation function determined by a calibration method. The calibration method includes determining coefficients of the evaluation function based upon known concentrations, and measured response data of calibration samples.