We present reactions of size-selected free silicon oxide cluster anions, SinO(m)(-) (n = 3-7, 2n - 1 = m = 2n + 2), with a CO gas. Adsorption of CO on SinO(m)(-) is observed as a major reaction channel. The rate constant of the adsorption reaction is high for the oxygen-rich clusters with m = 2n + 1, whereas almost no reaction product is observed for m = 2n. DFT calculations revealed that a pair of dangling O atoms on 4-fold-coordinated Si atoms plays a key role, which is the adsorption site of CO on SinOm-. Bond formation between CO and one of the dangling O atoms is associated with electron transfer from the CO molecule to the other dangling O atom. The present findings give molecular-level insights into adsorption of CO molecules on silicates in the interstellar environment.