A method for measuring venous oxygen saturation levels has steps of measuring optical absorption oscillation data at the respiratory frequency at a plurality of wavelengths (2). A reduced scattering coefficient and an absorption coefficient are determined for the tissue, with the result that an effective path length can be determined (6). Data processing is performed to calculate amplitudes for the absorption oscillation data that are translated into oxygenated and deoxygenated hemoglobin concentrations for the venous compartment (8). A method of the invention does not required mechanical ventilation devices or venous perturbation. Additional method steps may entail verifying that the measured absorption oscillation data results from the venous compartment.