Technical Abstract: Several types of evidence indicate that there is a significant resistance to the movement of carbon dioxide from the substomatal air space to the site of fixation in the chloroplasts and that the resistance may vary with temperature, carbon dioxide concentration and water stress. Methods of estimating the magnitude of this internal resistance are expensive and not readily available to most researchers or rely upon numerous assumptions which are difficult to prove. It is proposed that the measurement of the response of photosynthesis to oxygen, combined with a standard biochemical model of photosynthesis, can provide an estimate of the internal resistance. This method was used to determine whether the internal resistance changed with carbon dioxide concentration and with water stress in soybean leaves. Internal resistance was unchanged over the tested range of carbon dioxide concentrations for both unstressed and severely stressed leaves. Moderate water stress did not affect internal resistance in soybean, while severe stress reduced it. It is concluded that the internal resistance to carbon dioxide movement in soybean and presumably other species can be conveniently estimated using the response of photosynthesis to oxygen.