The complex behavior of bilirubin (BR) with beta-CD (cyclodextrin) and gamma-CD in aqueous and dimethylformamide (DMF) solution was investigated by absorption spectroscopy and cyclic voltammetry, respectively. The data shows that the complexation mechanisms in these two solvents are different. in aqueous solution, the complexes of BR-beta-CD and BR-gamma-CD were formed by internal inclusion reactions, while in DMF, they were formed by external, non-inclusion ways. The dissociation constants obtained for these complexes are K-d1 = 9.5 x 10(-4) M, K-d2 = 5.1 x 10(-4) M in aqueous solution and K-d3 = 12.5 x 10(-2) M, K-d4 = 1.73 x 10(-2) M in DMF, respectively.