Reactions of nickel oxide cluster ions, NinOn+x+ (n = 4-10, x = -1 similar to+3), with CO in a He buffer gas were investigated using mass spectrometry. When the cluster ions react at room temperature, a CO molecule tends to attach readily to NinOn+x+ for all of the cluster ions with different stoichiometry, although rate constants of the CO attachment reaction are more or less stoichiometry-dependent. However, CO was found to be released from the cluster ions when the cluster ions were heated up to 523 K after the reaction. This finding is interpreted such that the CO molecule that physisorbs weakly to NinOn+x+ at room temperature desorbs into the gas phase by the post heating. In addition, we found that an oxygen extraction reaction by a CO molecule actually occurs for oxygen-rich clusters such as Ni6O7+ and Ni8O9+. As the oxygen extraction reaction is one order of magnitude slower than physisorption, it lies hidden underneath of the faster physisorption processes in the mass spectrum but is revealed by the elimination of physisorption through the post heating.