The gas-phase hydroxyl(OH) radical emission was observed in the thermal decomposition of lithium hydroxide: (LIOH). This phenomenon was investigated in a vacuum flow tube reactor at around 2 Torr by the temperature programmed reaction (TPR) experiments at 500-1300 K. The production of OH and other gaseous products was quantitatively investigated by the laser induced fluorescence method and quadrupole mass spectrometry, respectively. The TPR spectra of OH had a peak at 1100-1200 K, which largely exceeded the gas-phase thermodynamic equilibrium. The origin of OH was supposed to be either the surface OH groups on Li2O or the residual LiOH in the LiOH/Li2O solid solution. OH production exceeding the thermodynamic equilibrium was explained by means of the partial equilibrium in the reaction: LiOH + 1/4O(2) <-> 1/2Li(2)O + OH. This phenomenon can be a new route for the OH production from H2O and O-2 in cyclic reactions of lithium compounds.