Hydrogen bond shortening and strengthening induced by confinement within fullerenes for a series of hydrogen bonded bihalides were studied. Significant increase in the electron localizability at the hydrogen bond region occurs as the confinement takes place. This result suggests that in confined systems there are additional effects that are beyond the 'electrostatic only' explanation offered for the existence of short and strong hydrogen bonds. We propose that these additional effects are factors on which the enhanced bonding strength energetic stabilization and basicity also depend. (C) 2010 Elsevier B. V. All rights reserved.