Changes in the proportion of hydrogen bonds of poly(4-vinyl phenol) (PVPh) and poly(methyl methacrylate) (PMMA) blends have been investigated by FTi.r. spectroscopy both above and below the grass transition temperature T-g. Changes in three spectral regions, namely the carbonyI, hydroxyl and aromatic ring vibration regions, are attributed to variations in hydrogen bonding with increasing temperature. Both hydrogen bonded O-H and C=O stretching vibrations shift to higher frequencies, indicating a weakening of interassociated hydrogen bonds at higher temperatures. A clear transition is observed in the absorbance behaviour of these vibrations at T,. The reduction in total area of O-H and C=O vibration regions is attributed not only to the breaking of hydrogen bonds at T-g, but also to the decrease of absorption coefficients of hydrogen bonded O-H and C=O stretching vibrations. Quantification of the decrease in concentration ratio of hydrogen bonds between carbonyI groups vs. total carbonyl groups showed a decrease of 0.5% in the number of hydrogen bonds was occurring at T-g + 30, when T-g is taken as the midpoint of the transition as determined by DSC, for the 30 wt% PVPh composition. For higher compositions, this percentage increased, reaching 3.5% for the 80 wt% PVPh composition, which is related to the increase in T-g of the blend.