Three enzymes (lysozyme, EC 3.2.1.17; -chymotrypsin, EC 3.4.21.1; and Candida rugosa lipase, EC 3.1.1.3) have been modified in order to alter their surface hydrophobic/hydrophilic balance in opposite directions, by chemoenzymatic glycosylation and chemical binding of polyethylene glycol (PEG). The thermal stability in aqueous environment of the produced biocatalysts was studied, and two different approaches were considered: the determination of half-life times and the mechanistic analysis of the deactivation kinetics. The comparison of half-life times indicated that an increase in enzyme surface hydrophobic character induced a remarkable amelioration in thermostability, while the increase in hydrophilic character produced the opposite effect. However, the investigation of kinetic and thermodynamic parameters of enzyme deactivation revealed, in some cases, secondary stabilisation effects during some step of the mechanism, wh alpha ich would not have been detected if only half-life times had been considered.