The isoelectric point of hemocyanin from Cancer is at about p H 4.8. The O2 dissociation curve has been determined at points ranging from p H 3.84 to 9.52. At about neutrality steepness of the dissociation curve is at a minimum. As acidity or alkalinity increases the curve approximates more and more closely a hyperbola. Shape of the curve appears to be controlled by relative amounts of pigment present as salt and in uncombined form.