The heat capacity of a liquid in which monomers chemically react to form a macromolecule has been measured as the reaction progresses under isothermal conditions. It first irreversibly increases and then undergoes a step decrease to a value which is mainly due to vibrational contributions. This is remarkably similar to the decrease in heat capacity observed near the glass transition temperature on supercooling a liquid. The number of covalent bonds in the macromolecule's structure at its vitrification decreases with the decrease in the isothermal reaction temperature. Conclusions on the role of intra- and intermolecular interactions determining the molecular kinetics of liquids have been drawn.