The hydrogen-bonding interactions and the crystallinity of biodegradable polymer poly(3-hydroxybutyrate) (PHB) blended with poly(4-vinylphenol) (PVPh) were studied on the as-cast films as a function of the blend composition w(pvph) by using Fourier-transform infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction. The intermolecular hydrogen bondings between the C=O groups of PHB and the OH groups of PVPh (designated as inter) were explored not only in C=O stretching vibration region but also in OH stretching vibration region. The bands in OH stretching vibration region were assigned. The spectra in the C=O stretching vibration region were decomposed into component spectra attributed to inter, the free C=O groups, and the intramolecularly hydrogen-bonded C=O groups between the C=O groups and one of the C-H groups in CH3 of PHB (designated as free and intro, respectively) to determine the respective fractions, f(inter), f(free), f(intra). We found that f(k)s (k = intra and inter) exhibit a double-step change with wpvph: (i) step 1 with 0 <= w(pvph) <= 0.5, where intro and inter exchange each other, when wpvph is changed, via the two elemental transformations, (a) from intro to free and (b) from free to inter, as will be discussed in detail in the text; (ii) step 2 with 0.5 < w(pvph), where X-ray crystallinity tends to vanisht, f(free) starts to decrease, and f(inter) starts to increase even more largely with wpvph than the case of (i), implying that the exchange from free to inter is suppressed by the presence of the crystallinity in step 1, while that from free to inter is enhanced in the absence of the crystallinity in step 2.