During the reversible reaction between peroxidase (HRP) and peroxides, several peroxidase intermediate species, showing different molecular absorption spectra, are formed which can be used for their determination. On this basis, a reversible reagentless optical biosensor based on HRP for hydrogen peroxide and peracetic acid determinations has been developed. The biosensor (which can be used for at least 3 months and/or more than 200 measurements) is prepared by HRP entrapment in a polyacrylamide gel matrix. A mathematical model (in which optical, kinetic and transport aspects are considered) relating the measured absorbance with the analyte concentration is also presented. Both peroxides show similar responses in the sensor film. Under the recommended working conditions, the biosensor shows linear response ranges from 6x10(-7) to 1.0x10(-4) M using FIA mode, and from 2x10(-7) to 1.5x10(-5) M using continuous mode for both peroxides; the precision, expressed as R.S.D., is about 4%. This biosensor has been applied for peroxide determination in waste water samples previously treated with peroxides.