The x-values in the nonstoichiometric chemical formulas $YO_{1.5+x};and;HoO_{1.5+x}$, have been measured in the temperature range from 700$^{circ}$C to 1000$^{circ}$C under oxygen pressures from $2{ imes}l0^{-1};to;1{ imes}10^{-6}$ atm by gravimetric method. The observed x-values increase with increasing temperature and oxygen pressure. The enthalpies of formation of excess oxygen in yttrium oxide and holmium oxide decrease with decreasing oxygen pressure and are all positive values representing an endothermic process. The 1/n values calculated from the slopes of the plots of log x vs. log $P_{O2}$ increase with temperature and are positive values which means the higher oxygen pressure dependence at higher temperature. We have examined the nonstoichiometric defect and conduction mechanism from x-values and thermodynamic data.