IR spectra of three H₂CO:H₂O ice mixtures at 15 K. The concentration ratio of each species in the mixture is determined from the integration of its characteristic IR bands. Mixture a) is a layered system where H₂CO is on top of H₂O, mixture b) is an homogeneous mixture of H₂CO:H₂O, and mixture c) is a layered system where H₂O is on top of H₂CO. The observation of small bands at around 3700 cm^-1, attributed to dangling OH, δ_OH, confirms that the ASW deposited at 15 K is porous.