The apparent depth of space charge region on the ZnO $(10{ar{1}}0)$ surface in chemisorption of oxygen has been estimated from the capacitance of two contacting faces. When the sample (donor concentration: $2.4{ imes}10^{22};m^{-3}$) was evacuated at 773 K for 1 hr the depth reached to 40-100 ${AA}$ depending on sample assembly. Admission of oxygen to the sample resulted in an increase of the depth to 3600 ${AA}$ where the increment was greater at higher oxygen pressure between 6.6-1600 $N/m^2$. Admission of CO to the sample previously exposed to oxygen yields a decrease in the depth. The results of the measurement support that oxygen is adsorbed as an acceptor on ZnO $(10{ar{1}}0)$.