The oxygen affinity, expressed as the P₅₀, of the haemocyanin of most cephalopods so far studied is relatively low (P₅₀ > 20 Torr in 75% of the studies). At the same time the Bohr effect, the dependence of oxygen affinity on pH, is large (Bohr coefficient < ‐1.0 in 80% of the studies).Spectrophotometric measurements of oxygen dissociation curves of the haemocyanin from Sepia officinalis at high oxygen tensions (>500 Torr) indicate that 100% saturation is not attained at low pH (6.5) compared to high pH (7.5). Cooperativity, expressed as n₅₀ also decreases with pH and these two characteristics suggest the presence of a Root effect, i.e. a dependence of maximum oxygen carrying capacity (O₂ cap) on pH. Oxygen dissociation curves at low pH however, were not asymptotic, indicating that full saturation may be reached at higher oxygen tensions. Gasometric measurements of oxygen carrying capacity using low pH and high PO₂ values (<650 Torr) also indicated the presence of a Root effect. However, high oxygen pressure measurements (>l atmosphere) in a specially constructed chamber show that 100% saturation was reached at higher oxygen tensions, indicating that this apparent Root effect is not due to an absolute conformational change, as in some fish haemoglobins, but to an exaggerated Bohr effect. Physiologically however, neither such high oxygen pressures nor such low pH values have been measured to date in the blood of cephalopods in vivo making the functional use of a Root effect doubtful.