The present invention provides a method for monitoring degradation of Hb-based blood substitutes, in a sample. This method involves determining a concentration of met-Hb within the sample, by applying a calibration algorithm for met-Hb to an absorbance obtained from the sample at one or more than one wavelengths, and using the concentration of met-Hb, as a measurement of degradation of the Hb-based blood substitutes. Using this assay, a concentration of met-Hb that is equal to or greater than 3% may be used as an indicator of degradation of Hb. Alternatively, by obtaining samples over a period of time, the concentration of met-Hb and the concentration of Hb-based blood substitute may be determined in each of these samples, and an increase in the concentration of met-Hb over the period of time is an indicator of degradation of Hb. The sample may be a whole blood sample, a serum sample, or a plasma sample obtained from a patient transfused with one or more than one Hb-based blood substitutes, a stock Hb-based blood substitute sample, or a body part of a patient.