Adsorption and diffusion of ammonia on an ice/Pt(111) surface were studied using Fourier transform infrared spectroscopy (FTIR) and thermal desorption spectroscopy (TDS). After exposing the crystalline ice film to ammonia molecules at 45 K, we have detected an intriguing feature at 1470 cm⁻¹ in the FTIR spectra. This feature is derived from the ammonia adsorption on the ice with a characteristic structure which appears in thin film region. The peak intensity decreases gradually as the thickness of the substrate ice increases. In addition, we have detected a feature at 1260 cm⁻¹ that appears after flashing the film to 113 K. The feature corresponds to the ammonia molecules which penetrate the ice surface and reach the ice/Pt(111) interface. Intriguingly, the intensity of this feature decreases with the ice thickness and there is a linear relation of the peak intensity of the features at 1470 cm⁻¹ and 1260 cm⁻¹. We propose a model that the solubility of the ammonia molecules is much higher for the thin ice film, whose structure is deformed from the ideal ice due to the presence of the metal substrate, than that for the ideal ice.