The entropy and related thermodynamic properties of methylisocyanate, CH3NCO, have been determined by isothermal calorimetry. The entropy in the ideal gas state at 298.15 K and 1 atmosphere is S-m(o) = 284.3 +/- 0.6 J/K . mol. Other thermodynamic properties determined include: the heat capacity from 15 to 300 K, the temperature of fusion (T-fus = 178.461 +/- 0.024 K), the enthalpy of fusion (Delta H-fus = 7455.2 +/- 14.0 J/mol), the enthalpy of vaporization at 298.15 K (Delta H-vap = 28768 +/- 54 J/mol), and the vapor pressure from fusion to 300 K. Using statistical thermodynamics, the entropy in this same state has been calculated for various assumed structures for methylisocyante which have been proposed based on several spectroscopic and ab initio results. Comparisons between the experimental and calculated entropy have led to the following conclusions concerning historical differences among problematic structural properties: (1) The CNC/CNO angles can have the paired values of 140/180 degrees or 135/173 degrees respectively. It is not possible to distinguish between the two by this thermodynamic analysis. (2) The methyl group functions as a free rotor or near free rotor against the NCO rigid frame. The barrier to internal rotation is less than 2100 J/mol. (3) The CNC vibrational bending frequency is consistent with the more recently observed assignments at 165 and 172 cm(-1) with some degree of anharmonicity or with a pure harmonic at about 158 cm(-1). (C) 2012 Elsevier Ltd. All rights reserved.