A method for quantitative analysis of hemoglobin color to estimate the oxygen saturation was developed. The method uses an image-input and -processing system composed of a 3-tube video camera and a digital image analyzer. Using the system connected to a microscope, facilitated diffusion of oxygen in hemoglobin solutions was observed and analyzed in a position-sensitive way. The results confirmed its applicability to this study and gave information about the diffusion mechanism expressed by the empirical formula J = kY, where J is the flux of oxygen, Y is the oxygen saturation of hemoglobin, and k is a constant.