The possibility of measuring the blood oxygenation in the brain with near infrared light has been studied. The goal of this study was to quantify the influence of different brain layers on brain blood oxygenation measurements. Experimental results obtained from time resolved reflectance measurements on layered tissue phantoms were compared to Monte Carlo simulations of layered models, diffusion theory, and in vivo measurements on the human head. Both the experimental results and simulations show that the absorption coefficient μ a, which is closely related to the blood oxygenation, of deeper layers can be accessed in the time domain. Thus fitting analytical expressions found from diffusion theory only to the late part of the time resolved reflectance allows us to determine μa and subsequently the blood oxygenation of the deepest medium (e.g. brain tissue).