In the early 1990s your authors believed that they had essentially solved the problem of predicting the phase behaviour of hydrogen-bonded polymer blends. A text devoted to this subject, Specific Interactions and the Miscibility of Polymer Blends, was published and we thought that it was now time to look around for something else to do. This was before a colleague, Boris Veytsman, pointed out that there was a flaw in our derivation of the free energy of mixing equation. It has taken us some 15 years to correct the theory and match the predictions of the phase behaviour of hydrogen-bonded blends that we presented in our 1991 book. So we have come full circle. The subject has become far more complicated, but at the same time far more interesting. Along the way we have discovered that the phase behaviour of hydrogen-bonded polymer blends can be successfully predicted using equilibrium constants determined from appropriate low molecular weight analogues, if chain connectivity effects such as intermolecular screening and functional group accessibility are included.