The carbon monoxide (CO) adsorption activity of small gold-cluster anions (Au-N(-), N = 4-19) has been measured by pulsed helium flow-reactor methods at room temperature. In contrast to other metals, the reaction-product distributions, Au-N(CO)(M) = (N,M), measured over a range of CO partial pressure, reveal highly size-dependent adsorption activity and saturation. Preferred initial products include the (N,M) set {(5,1), (11,1), (15,1). (15,2)}, which can be explained by the gold electron (sub)shell fillings at 8, 14, 18, and 20 electrons, respectively, assuming CO acts as a two-electron donor. Pronounced saturation compositions include the complexes {(7,4), (9,6), (11,6), (13,6)}. The pressure-dependent adsorption patterns give insight into (CO,O-2) coadsorption phenomena on model-catalyst systems, and may be exploited in further gas-phase experiments on coadsorption and catalytic cycles.