We investigate the potential of CW fNIRS with two NIR wavelengths to estimate tissue oxygen saturation by a model based on spatially resolved spectroscopy. Phantoms with optical properties similar to brain tissue are applied to study the influence of the optical parameters and the spatial probe configuration on effective attenuation, absorption, and oxygen saturation. Attenuation coefficients and saturation values can be estimated with a typical discrepancy of 5%, whereas absorption coefficients show larger deviations from the true values. The results are very sensitive against small errors in the source-detector separation.