The equilibrium constants describing phenolic OH/ester carbonyl hydrogen bonds are experimentally determined for polymer blends, mixtures of low molecular weight analogues of the polymer repeat units, solutions of the polymers in the low molecular weight analogues, and random copolymers of the original blend segments. These equilibrium constants are determined by infrared spectroscopy, and it is demonstrated that the interactions in the low molecular weight analogues and polymer solutions are (within error) described by the same equilibrium constant. The blend equilibrium constant has a value that is much smaller (about 25% of the solution value), while the copolymer value is intermediate between these two extremes. We propose that this large difference is due to compositional heterogeneities in the blend.