<jats:p>Address for reprint requests.</jats:p> <jats:p>The in vitro oxygen-binding characteristics of haemocyanin were investigated in whole blood of two species of pelagic squid, Illex illecebrosus and Loligo pealei. pH-independent Haldane coefficients (ΔHCO3−/ΔHcyO2) (where HcyO2 is haemocyanin-bound oxygen) slightly smaller than —1 were found in both species. Oxygen-linked CO2 binding was not present. Buffer values ranged between 5 and 5.8 m mol l−1pH unit−1. For further analyses a pH/saturation diagram was selected to show the effect of pH on oxygen binding at constant POO2 in a continuous plot. The slopes of the resulting oxygen isobars (ΔHcyO2/ΔpH or ΔS/ΔpH) (where S is oxygen saturation) depend on pH. The diagram allows evaluation of both the Bohr coefficients (ΔlogP50/ΔpH) and the Hill coefficients (n50) at specific pH values. It provides an integrated illustration of the importance of the Bohr effect and cooperativity for oxygen binding.</jats:p> <jats:p>In accordance with Wyman's linkage equation, Bohr and Haldane coefficients are found to be identical. Both are pH-independent between pH7 and 8. The changing slopes of the oxygen isobars are likely to reflect changes in cooperativity with pH. Maximum values of n50 coincide with maximum steepness of the oxygen isobars in the physiological range of pH and POO2. Assuming that the haemocyanin acts as a buffer for venous POO2, this maximum in pH sensitivity and its decrease in the higher and lower pH ranges are discussed in the light of the maintenance of pigment function in vivo.</jats:p>