A detailed experimental study of the infrared characteristics of CO in H2O ice matrixes (10-150 K) has been performed. The effects of thermal and ion irradiation processes have also been examined. H2O:CO ice mixtures have been obtained both by co-deposition of H2O and CO (at 10 It) and by diffusion (at similar to 27 K) of eo into a preformed water ice him. It has been confirmed that CO does not diffuse into the ice at temperatures below similar to 26 K. Results have been compared with previously reported diffusion experiments at 42 K. Laboratory experiments show that the profile (shape, width, and peak position) of the CO band depends on the structure of the water matrix it is embedded in and on the process (co-deposit or diffusion) by which the mixture has been obtained. This is attributed to Be different amount of energy involved in the process which forms the mixture. These studies are important to understand the physical and chemical properties of ices. Furthermore information on the history of molecular ices in astrophysical environments are obtained comparing observed spectra with those taken in laboratory.