Hydrogenation of organic chemicals is one of the most frequent things that people take for granted in mass spectroscopy; however, it could provide important information on spontaneous or stimulated hydrogen transfer in initiating chemical reactions and in determining the product selectivity and conversion efficiency. Here, we present a study of hydrogenation of acetone via vacuum ultraviolet laser ionization mass spectrometry (VUV-LIMS) and density functional theory (DFT) calculations. It is interestingly found that acetone dimer readily captures a hydrogen to form (C3H6O)(2)H+ in the presence of alcohols, shedding light on the intracluster hydrogen atom transfer via a trimolecular mechanism. This is well consistent with the DFT calculation results of energetics and reaction kinetics. It is worth noting that, although the hydrogen bond interaction of O-H center dot center dot center dot O is stronger than that of C-H center dot center dot center dot O, the hydrogen atom transfer (HAT) tends to proceed from the methyl group of the alcohols to acetone. We fully demonstrate the intracluster HAT reactivity of such a simple system and provide new insights into hydrogen bond interactions and molecular cluster chemistry.