There are two different adsorption states of CO on amorphous solid water (ASW): one adsorption state consists of CO interacting with non-hydrogen-bonded (dangling) OH, while the other consists of CO interacting with bonded OH. Photoirradiation onto ASW at 157 nm induces CO2 formation through the reaction of CO with OH, where OH originates from the photodissociation of H2O ice. The preferential formation of CO2 would explain the missing infrared CO band in the observation of interstellar ice particles.