We present results from laboratory experiments on layered CO–H2O-ice systems, carried out from sub-monolayer to multilayer CO coverages, and review recent experimental data, as published by the authors. Under certain specific laboratory conditions the 2152 cm−1 feature, associated with CO molecules adsorbed at dangling-OH bonds at the ice surface, is ‘missing’. A detailed analysis is used to understand why the same feature is not detected in spectra of interstellar ices. We conclude that the dangling-OH sites do exist in interstellar ices but that the sites are blocked by another species. The astronomical implications of this deduction are discussed.