The heat capacity of poly-p-dioxanone (PPDX), (CH2‒CH2‒O‒CH2‒COO‒)x, was determined using both differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) from 200 K to 430 K. Based on the new data and literature data, the heat capacity of the solid state was analyzed using an approximate group vibrational spectrum and skeletal vibrations. The 10 skeletal vibrational modes are well represented by a Tarasov function with theta temperatures of1= 478.7 K and3= 50.4 K. The heat capacity of the liquid was fitted to a linear function,Cliquid= 0.1484T+ 144.3 in units of J K−1mol−1, which is close to the sum of equations developed earlier for the liquids of poly(oxyethylene) and polyglycolide. The change in heat capacity of amorphous PPDX at the glass transition temperature (264 K) is 69.9 J K−1mol−1, and the heat of fusion for perfect crystals at the melting temperature (≈400 K) is 14.4 kJ mol−1. The integral thermodynamic functions were derived, and the residual entropy of the glass at 0 K, determined from these functions, is extrapolated as 13.4 J K1mol−1(or 2.7 J K−1mol−1per chain atom).