The adsorption and desorption of CO on and from amorphous H 2 O ice at astrophysically relevant temperatures has been studied using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Solid CO is able to diffuse into the porous structure of H 2 O at temperatures as low as 15 K. When heated, a phase transition between two forms of amorphous H 2 O ice occurs over the 30–70 K temperature range, causing the partial collapse of pores and the entrapment of CO. Trapped CO is released during crystallization and desorption of the H 2 O film. This behavior may have a significant impact on both gas-phase and solid-phase chemistry in a variety of interstellar environments.