The thermal denaturation of hen egg white lysozyme (HEWL) in D2O was followed by IR absorption after addition of dimethyl sulfoxide (DMSO) at different Molar. fractions. Amide I intensify and position revealed that DMSO reduces. the thermal. stability of the native protein and favors the formation of ordered aggregates: The comparison with ethanol/water solutions evidenced that ethanol (partially deuterated ethanol EtOD) has a stronger effect on the thermal stability of HEWL the same down-shift of melting temperature was measured at 0.18 and 0.30 molar fraction of ethanol and DMSO, respectively. This is probably due to lower polarity of EtOD/D2O with respect to DMSO/D2O solutions. A kinetic Study of protein assembling at 0.30 DMSO molar fraction, was also performed at different temperatures. The high viscosity of the solvent was observed to cause a sensitive slowing down of aggregation rate in comparison to that of water/alcohol solutions. The evidence of a retarded self assembling put forward a possible explanation for the use of dimethyl sulfoxide as a protectant of protein structure In fact, for both organic solvents a nonspecific interaction with the protein and a water mediated action Is deduced, but the addition of DMSO reduces the irreversible denaturation due to kinetic effects and this can be exploited for lessening one of the Main degradation routes of globular proteins during freezing-thawing cycles.