1. 1. The oxygen affinity of purified Katharina tunicata hemocyanin was measured under a variety of conditions of salinity, temperature and pH. 2. 2. Purified hemocyanin shows a cooperative oxygen binding curve ( n H ranges from 1.6 to 3.0) and a normal Bohr effect ((Ø ranges from −0.080 to −0.199) at the various ion concentrations and temperatures examined. 3. 3. Oxygen affinities of hemocyanin in a buffer with inorganic ions identical to those measured in animals exposed to 30%. and 10%. sea water (pH 7.4) were slightly lower at the higher salt concentration. When the salinity of the buffer was increased to 35%. or 45%. the oxygen affinity of the hemocyanin was further lowered, most strikingly at 25°C. 4. 4. The cooperativities and Bohr coefficients at high salinities were similar to those at lower salinities. 5. 5. The heat of oxygenation, Δ H, of Katharina hemocyanin was about −9.4kcal/mole (pH 7.4). 6. 6. Several organic molecules, succinate, d-lactate, alanine, propionate and acetate, show little or no effect on hemocyanin oxygen binding at either 30%. or 10%. 7. 7. The oxygen affinity of Katharina hemocyanin could change two fold over salinity and temperature conditions which the animal experiences in the field.