There is increasing observational evidence that the composition of interstellar dust varies with interstellar environment. Irradiation from UV photons and cosmic rays, as well as chemical interactions with the interstellar gas, play a crucial role for grain transformation. The analysis of "laboratory analogues" represents a powerful tool to better understand the nature and evolution of cosmic materials. In particular, the study of grain processing under simulated dense and diffuse interstellar conditions is fundamental to outline an evolutionary pathway for interstellar dust.

We review the results of experiments aimed at studying the variations of the composition of nano-sized carbon particles in response to UV and ion irradiation and H atom exposure. These results give us the opportunity of reconciling the dichotomy of the 3.4 μm band in the spectrum of diffuse and dense clouds through an evolutionary transformation of the aliphatic component caused by grain processing. Moreover, we discuss formation of CO and CO₂ molecules by ion irradiation of water-ice covered hydrogenated carbon grains. This result indicates that the composition of the refractory component can influence the chemical evolution of ice mantles during energetic processing in dense clouds.