The binding of various ligand molecules to the binuclear Cu(I) site of deoxy-hemocyanin has been investigated through the changes produced in the aromatic region of the circular dichroism spectrum of the protein, where a cluster of tryptophan residues located in the vicinity of copper site undergo conformational reorientations in the presence of exogenous ligands coordinated to the metal. In agreement with expectations, the binuclear site of arthropod hemocyanin is severely hindered to the access of exogenous ligands except for very small molecules like CO, O2 or CN- while for mollusc proteins ligands such as thiourea and 2-mercaptoethanol bind easily to the Cu(I) sites. However, the access of the ligand becomes progressively hindered and eventually prevented as the size of substituents on the ligand increases.