The effect of hydrogen bonding on the free volume in polymer blends is reported using infrared spectroscopy and positron annihilation lifetime spectroscopy (PALS). The system under investigation comprised blends of poly(N-vinylpyrrolidone) with poly(vinyl acetate-co-vinyl alcohol) containing varying ratios of vinyl acetate:vinyl alcohol. Infrared spectroscopy revealed the presence of hydrogen bonding in the blends as a shift to lower wavenumbers of the pyrrolidone carbonyl and the alcohol hydroxyl bands. PALS revealed an increase in free volume, in comparison to the individual components, of the polymer blends with increase in the hydrogen bonding, but this is followed by an unexpected "collapse" of the network structure above a critical value of hydrogen bonding.