The effects of pressure on the binding of a hydrophobic compound (8-anilina-1-naphthalenesulfonate (ANS)) to poly(vinylpyrrolidone) (PVP) in aqueous solution were followed by the fluorescence enhancement over the pressure range 0.1-400 MPa. An elevation in pressure induced the dissociation of ANS from an oligomer PVP (molecular weight <6000) but favored its binding to a larger PVP molecule (MW > 10 000). We calculated the binding constant between ANS and PVP (K-b) and estimated the apparent volume changes accompanying the ANS binding to PVP (Delta V-b) from the pressure dependence of K-b. The volume changes for ANS binding to high-MW PVP＇s were dependent on the pressure, and negative values were obtained in the pressure region from 100 to 400 MPa. SAXS measurements demonstrated that the ANS binding to the high-MW PVP chain led to the reduction of radii of gyration, suggesting that ANS binding induced local cluster formation in the PVP chain around the binding site. The Kratky plot indicated that global shrinkage of the chain occurred, and the negative volume change for ANS binding to high-MW PVP is explained by the fact that the cluster formation around the ANS is followed by an inclusion of the cluster around ANS into the random coil for PVP.