Low-temperature heat capacities and high-temperature enthalpies were determined calorimetrically for na2cro4. A reversible transition was found at 700 deg k with an isothermal heat of transition of 2,263 cal/mole. High-temperature x-ray diffraction analysis showed that the transition was a crystal change from the low-temperature orthorhombic form to the high-temperature hexagonal form. At 795 deg k the hexagonal form had the parameters ao = 5.691 and CO = 7.478 A. Powder diffraction data for the high-temperature form are given and possible structures discussed. The melting point was determined to be 1,065 deg k with a heat of fusion of 5,810 cal/mole. Enthalpy and entropy increments from this investigation were combined with other data from the literature to calculate enthalpies of formation, free energies of formation, and equilibrium constants for formation from 298 deg to 1,400 deg k. Equations that represent enthalpies above 298 deg k are given. Work done under an agreement with the U.S. Atomic Energy Commission.