A notable hysteretic effect has been observed in the interaction of Co(Ⅱ) with human serum albumin (HSA) or bovine serum albumin (BSA) using UV-Visible spectrometry at physiological pH (7.43), which shows that the binding between Co(Ⅱ) and HSA or BSA may induce a slow transition of HSA or BSA from the conformation of weaker affinity for Co(Ⅱ) to one of stronger affinity (A-B transition). The rate constants and activation parameters of this transition were measured and are discussed. It is inferred that such a conformation transition may occur due to the binding of the first Co(Ⅱ) ion with the peptide segment of N-terminal residues 1-3, which results in a 'hinged movement' of the relatively hydrophobic 'valley' in the IA subdomain. This process leads to a slow conformational transition in the albumins, makes the other binding sites of Co(Ⅱ) exposed, and shows a positive cooperativity effect. The LMCT (ligand-to-metal charge transition) bands of the Co(Ⅱ)-HSA and Co(Ⅱ)-BSA systems also show a kind of hypochromic effect featuring a dipole-dipole interaction mechanism. This phenomenon is rarely reported.