The thermal behaviour of some commercial sodium hyaluronate solutions used in ophthalmology has been studied by DSC after cooling to -100 degrees C. During the rewarming, the endothermic effect that appears at around -22 degrees C is associated with the molecular weight (MW) of the fractions of sodium hyaluronate : the higher the MW, the higher the endotherm temperature. At high temperatures, the dehydration temperatures of the samples (onset at around 110 degrees C) are in a sequence that is the inverse of that obtained at low temperatures. Both effects, low-temperature phase transition and dehydration, have been related to intramolecular and intermolecular hydrogen bonding capabilities and to hydration and crosslinking effects.