An analysis in terms of elastic scans of the neutron intensity of homologous disaccharide (trehalose, maltose, sucrose)/H2O mixtures as a function of temperature and exchanged wave vector has been carried out. The experimental findings, showing a crossover in molecular fluctuations between harmonic and anharmonic dynamical regimes, allow us to characterize the system "flexibility.'' A new operative definition for the "fragility'' degree, by using elastic incoherent neutron scattering, is furnished. In this frame the lower flexibility and fragility character of trehalose/H2O mixture with respect to maltose and sucrose/H2O mixtures indicate a better attitude to encapsulate biostructures in more rigid and temperature insensitive structures in approaching the glass transition. (C) 2003 American Institute of Physics.