A simple method to evaluate the hemoglobin oxygen saturation and relative hemoglobin concentration in a tissue from diffuse reflectance spectra in the visible wavelength range is put forward in this paper. It was assumed that while oxygenated and deoxygenated hemoglobin contributions to light attenuation are strongly variable functions of wavelength, all other contributions to the attenuation including scattering are smooth wavelength functions and can be approximated by Taylor series expansion. Based on this assumption, a simple, robust algorithm suitable for real time monitoring of the hemoglobin oxygen saturation in the tissue has been derived. This algorithm can be used with different fiber probe configurations for delivering and collecting light passed through the tissue. An experimental technique using this algorithm has been developed for in vivo monitoring during artery occlusion and in vitro monitoring of blood samples. The experimental results obtained are presented in the paper. © 2001 Society of Photo-Optical Instrumentation Engineers.