The binding mechanism of 6,6-[(3,3-dimethyl [I,I-biphenyl]-4,-4 diyl) bis(azo)] bis [4-amino-5-hydroxy-l ,3-naphthalene disulfonic acid] tetra sodium salt, usually called Evan blue (EB), to poly (N-vinyl 2- pyrrolidone) (PVP) in aqueous solution has been studied spectrophotometric ally at pH 9.2 in phosphate buffer in the presence and absence ofNaCI. Am •• = 606 nm for EB solutions was observed and then by the addition of PVP red shifted to 636 nm. Analysis of results by Scatchard and Hill methods showed a mixed cooperativity mechanism at different [PVPj. Thus at high [PVP] there is positive cooperativity in the conversion of weak to strong complex and negative cooperativity at low [PVP]. The V-shaped Scatchard plot at fixed [PVP] and varying [dye] further supplements the operation of positive and negative cooperativity.