Hemocyanins are large oligomeric respiratory proteins found in many arthropods and molluscs. Here we give infrared spectroscopic evidence of a high stability towards exposure to sub-zero temperatures for hemocyanins from the arthropods Limulus polyphemus and Eurypelma californicum at different pH values. Small but distinct temperature induced changes of the secondary structure were observed, but a stable core of at least 40% α-helical structure is preserved as identified in the infrared spectra obtained between 294 and 20 K. The structural changes differ in detail somewhat for the two hemocyanins, with overall fewer changes observed in the case of E. californicum. Notably, in both cases the overall changes in the α-helical content are found to be fully reversible. The small changes in the secondary structure and reversibility upon cold treatment seem to be a particular property of the two hemocyanins, since it was not observed for myoglobin studied in the same way.