The authors have made a calorimetric study of the thermodynamic properties of p-methacryloyloxybenzoic acid and poly- p-methacryloyloxybenzoic in the region 0–500 K. They have measured the thermal capacity C p 0, the temperatures and enthalpies of the physical transitions and determined the enthalpies of combustion and polymerization of p-methacryloyloxybenzoic acid. From the results they calculate the functions H 0 ( T) - H 0 (0), S ( T) and G 0 ( T) - H 0 (0) of the monomer and polymer for the region 0–330 K and also δH c 0, δH f 0, δS f 0 and δG f 0 of these substances for standard T and p. For the region 0–500 K they calculate the enthalpy, entropy and Gibbs energy of polymerization of p-methacryloloxybenzoic acid in bulk at p=101·325 kPa for different physical states of the monomer and polymer. The upper limiting temperature of the process is evaluated.