Reactions of aliphatic isocyanates with a phenolic ester alcohol (PHEA) were investigated using C-13-NMR spectroscopy. PHEA has two reactive sites: a phenolic -OH group and a secondary aliphatic -OH group. Both -OH groups react with the isocyanate groups. With an organotin catalyst, dibutyltin dilaurate (DBTDL), the aliphatic -OH group reacts first. With a tertiary amine catalyst, 1,4-diazabicyclo[2.2.2]octane (DABCO), or triphenylphosphine (Ph3P) or even in the absence of a catalyst at room temperature (RT) the phenolic -OH group reacts first. With the organotin catalyst, the reactions are generally complete in a day at RT. With DABCO or triphenylphosphine or DNNDSA catalysts, the reactions are almost complete only in 3-4 days at RT in ethyl acetate or acetonitrile. Uncatalyzed reactions are slower. With an acid catalyst such as dinonylnaphthalenedisulfonic acid (DNNDSA), both -OH groups react with the isocyanate. When equimolar quantities of PHEA and hexamethylenediisocyanate (HDI) polymerize at RT or reflux in the presence of a catalyst, both -OH groups react, with the phenol reacting slowly. Upon refluxing, the phenolic -OH-based urethane slowly rearranges (transcarbamoylation) to the aliphatic -OH-based urethane. DABCO and Ph3P catalysts effect this rearrangement at a much slower rate than does the acid catalyst. In the presence of a catalytic amount of DBDTL in a refluxing solvent, this rearrangement is complete in 2 h. By refluxing the phenolic-OH-based urethane in isopropanol, the mechanism of transcarbamoylation was found to be intermolecular. The mechanism is likely to involve deblocking of the phenolic urethane and subsequent reaction of the isocyanate generated, with the aliphatic -OH group. This conclusion was confirmed by differential scanning calorimetry (DSC) experiments.