The data on thermodynamic and molecular properties of the lithium and sodium hydroxides have been collected, critically reviewed, analyzed, and evaluated. Tables of thermodynamic properties (C°p,&Fgr;°=−(G°−H°(0)/T,S°,H°−H°(0), &Dgr;fH°, &Dgr;fG°) of these hydroxides in the condensed and gaseous states have been calculated using the results of the analysis and some estimated values. The recommendations are compared with earlier evaluations given in the JANAF Thermochemical Tables and Thermodynamic Properties of Individual Substances. The properties considered are: the temperature and enthalpy of phase transitions and fusion, heat capacities, spectroscopic data, structures, bond energies, and enthalpies of formation at 298.15 K. The thermodynamic functions in solid, liquid, and gaseous states are calculated fromT=0 to 2000 K for substances in condensed phase and up to 6000 K for gases. ©1996 American Institute of Physics and American Chemical Society.