FTi.r. was used to investigate quantitatively hydrogen bonds in poly(methyl methacrylate) (PMMA) and poly(4-vinyl phenol) (PVPh) blends. An absorption coefficient of free (or non-hydrogen bonded) carbonyl vibration of 750 +/- 40 cm(-1) m(2) mol(-1) was determined from pure PMMA using the Beer-Lambert law, along with precise measurements of film thickness and density. By making the approximation that this value is the same for pure PMMA and for PMMA in blends, the absorption coefficient of the hydrogen bonded carbonyl vibration was calculated as 910 +/- 50 cm(-1) m(2) mol(-1). Using these values, it was deduced that less than 45% of carbonyl groups are involved simultaneously in hydrogen bonding for a blend where the PVPh to PMMA mole ratio was 1 to 1. In pure PVPh, O-H groups exist as n-mers, with a predominance of shorter mers such as trimers or tetramers. Upon addition of PMMA to PVPh, the fraction of intra-associated O-H bonds remains, within experimental error, constant whereas inter hydrogen bends between PVPh and PMMA chains increase, resulting in a decrease in free O-H groups. Above 45.5 wt% PMMA, almost all ＇free＇ O-H groups have disappeared, and intra-and interassociated hydrogen bonds are distributed among the available OH groups. Finally, at high PMMA content (90 wt% PMMA, or 10 wt% PVPh), the probability of two PVPh chains to be close enough to allow intrahydrogen bond formation is small, and only inter-hydrogen bonds between PVPh and PMMA are observed.