Abstract: A portable spectrophotometric apparatus and method therefor for measuring the percentage of carboxyhemoglobin (COHb) in blood. The apparatus comprises a spectrophotometer and wavelength selection filters for testing a reference sample having a hemolizing agent and sodium hyposulfite and a treated blood sample with the hemolizing agent and sodium hyposulfite and providing a two component COHb-Hb system for determining relative absorbance values of the two samples at light wavelengths of 420 nanometers and 432 nanometers, the wavelengths corresponding to respective peak absorbance of the components in the high absorbance Soret region on opposite sides of an isobestic point formed by the intersection of their respective absorbance-wavelength curves. The ratio of the derived relative values is compared against predetermined data to determine the percentage of carboxyhemoglobin in the blood.