Abstract: A method for determining myoglobin oxygen fractional saturation in vivo in muscle tissue and intracellular oxygen tension (pO.sub.2), typically in the presence of hemoglobin, is provided. The method comprises measuring the absorption spectrum of the tissue using spectrographic equipment known to the art, including equipment for non-invasively taking spectroscopic measurements of tissue, or adaptations of such equipment to provide the preferred measurements described herein, or using spectrographic equipment specifically designed as described herein for use in non-invasive measurements of reflectance spectra. The measured spectrum is corrected for light scattering effects, such as by taking the second derivative of the data, or by using other means which are known to the art.