The fully amorphous films of highly syndiotactic poly[(R,S)-3-hydroxybutyrate] (s-PHB)/atactic poly(4-vinylphenol) (PV-Ph) blends show reversible thinning/ thickening phenomena at 37 degreesC in aqueous medium. On the other hand, isotactic poly[(R)-3-hydroxybutyrate] (i-PHB)/PVPh blend film, in which i-PHB blend component was partially crystalline, did not show any thinning/thickening phenomena under the same conditions. To elucidate the factors influencing these phenomena, the structure and molecular interaction in these blends were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry, and wide-angle X-ray diffraction. The FTIR spectra indicated that the ester carbonyl of PHB and the phenolic hydroxyl of PVPh formed hydrogen bonds in both the thinned and thickened s-PHB/ PVPh blend films. The blend composition, intermolecular hydrogen-bonding interaction, crystallization behavior, miscibility, and the glass-transition temperature of the blends affected the thinning/thickening phenomena. Some other polyesters such as poly(epsilon-caprolactone), poly (L-lactic acid), atactic poly(D,L-lactic acid), and poly(ethylene terephthalate) had no ability to exhibit thinning/thickening phenomena in water at 37 degreesC when they were blended with PV-Ph. This result implies that s-PHB/PV-Ph is the rare example with the ability to show reversible thinning/thickening phenomena.