The heat capacity of a polycrystalline sample of MnO was measured from T approximate to 1 K to T approximate to 400 K using two different experimental apparatuses. Features revealed by the data include a hyperfine contribution due to the Mn nuclei, a T-2 temperature dependence at low temperatures due to the type II antiferromagnetic magnon contribution, and a sharp but well-defined antiferromagnetic transition with T-N = 117.7095 K. The third-law entropy at T = 298.15 K, Delta(0)(298.15) S-m(o), calculated from these heat capacities is 59.02 J.K-1.mol(-1), which is 1.2 per cent lower than the previously accepted value. Revised values for Delta(f)H(m)(o) and Delta(f)G(m)(o) of MnO are tabulated up to T = 950 K.