The binding of oxygen to hemoglobin is thought to be a multistep process with different rate constants for each step. With an all tantalum stopped-flow calorimeter developed earlier, we can easily measure the heat of binding of oxygen to hemoglobin. The all-tantalum system will prevent the diffusion of oxygen into the system and will allow us to measure these heats over the entire saturation range, 0-100%; preliminary measurements of the binding heat at 50% saturation are in good agreement with published values.