We present experimental results obtained by IR spectroscopy in the 1.75--25 μm (5700--400 cm-1) range of ice mixtures containing nitrogen, water, and methane. Some of the studied mixtures have also been irradiated with energetic ions and their spectra collected at different temperatures. Irradiation produces molecules not present in the original mixture and leaves over a complex refractory residue whose color, because of progressive carbonization, becomes darker and darker as the irradiation dose increases. The results are discussed with a view of their relevance to the chemistry of the surfaces of Pluto and Triton. It is suggested that isolated water molecules should be searched for; many not yet detected molecular species (in particular some containing CN groups) should be present. A hypothesis to explain the observation of CO2 on Triton (but not on Pluto) is also proposed. ¿ 2001 American Geophysical Union