The optical absorbance spectra of oxy-, carboxy-, and deoxyhemoglobin were recorded at wavelengths from 479 to 651 nm and at temperatures of 20, 30, and 40 degrees C. As noted in earlier reports, a major effect of lowering the temperature was an increase in the absorptivities at or near the absorbance maxima. However, at other wavelengths, reducing the temperature increased, decreased, or caused no change in absorbance. At wavelengths where temperature-induced shifts did occur, the absorbance change appeared to be a linear function of temperature. Unlike previous reports, the data presented here are quantitative and thus can be used to predict temperature-induced errors in spectrophotometric measurements of the relative concentrations of these hemoglobin species. Examples are given of the error that would occur in a widely used CO-Oximeter, the IL482, if it were not temperature controlled. Thus, the data presented here should be particularly useful to the operators and designers of spectrophotometric instruments such as oximeters, CO-Oximeters, and hemoglobinometers.