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Here, we report results of spectrophotometric, spectrofluorimetric, and ultrafiltration studies on the reaction between cobinamides (viz. aquahydroxo-, nitro-, aquacyano- and dicyanocobinamides; Cbi) and bovine serum albumin (BSA), and reactivity of formed complexes toward cyanide. (H2O)(HO−)Cbi binds BSA at almost equimolar ratio predominantly via amino group of lysine side chains. The mechanism of the reaction involves two steps, i.e. the coordination of amino group on Co(III), and further stabilization of the generated complex. The reaction of (H2O)(NO2−)Cbi with BSA is similar with that involving (H2O)(HO−)Cbi, and both complexes bind cyanide significantly slower than free (H2O)(HO−)Cbi and (H2O)(NO2−)Cbi. (H2O)(CN−)Cbi binds BSA predominantly via aminogroup as well, however, its coordination proceeds substantially faster and less tightly than in the case of (H2O)(HO−)Cbi. Binding of (H2O)(CN−)Cbi and (H2O)(HO−)Cbi occurs at different sites of BSA as was indicated by spectrofluorimetric titration. Reaction of the complex between (H2O)(CN−)Cbi and BSA with cyanide proceeds much faster than in the case of the complex between (H2O)(HO−)Cbi and BSA. (CN−)2Cbi is partially decyanated by BSA, however, its binding by BSA is relatively low.