The binding of cyanide ion to Fe(II1) in the hemes of bovine methemoglobin and insoluble polyion complex (KPVS-Hb-PDDA complex) prepared by the complexation of the methemoglobin with potassium poly(viny1 alcohol) sulfate and poly(diallyldimethy1ammonium chloride) was investigated as functions of pH and potassium cyanide concentration by spectrophotometric method and adsorption experiment. The degree of saturation of the cyanide ligand on the heme showed a maximal value at pH range 8-9, whereas this was reduced to zero in the strongly acidic and basic regions, below pH 3.5 and above pH 13.5. These results were in agreement, in a qualitative way, with the theoretical results represented by four equilibrium reactions between methemoglobin and potassium cyanide at different pH and cyanide concentrations. The separation of cyanide ion in Na2B407-KH2P04 buffer solution at pH 9.0 was also investigated by using a KPVS-Hb-PDDA complex column. The cyanide ion in the solution was mostly or entirely bound to the complex until the heme was saturated with cyanide ion, and most of the bound cyanide ion was eluted with 0.1N NaOH solution. These results indicate that KPVS-Hb-PDDA has a function as cyanide ion exchanger and that the exchange reaction is analogous to that for methemoglobin.