Phytase is used as an animal feed supplement to enhance the nutritive value of plant material by liberation of inorganic phosphate from phytic acid (myo-inositol hexakisphosphate) and, thereby, to reduce environmental phosphorus pollution. For feed application a thermostable phytase is of general interest to circumvent problems that may occur during the formulation and feed pelleting process where temporarily high temperatures (80-100 °C) and shear stress may affect protein structure and lead to activity loss. To study different physicochemical rationales for stabilization of enzymes against thermal inactivation, we have used Aspergillus fumigatus phytase as a model enzyme. Subsequently, the stabilization concepts were tested on other fungal (A. nidulans, A. terreus, and A. niger phytase) and consensus phytases. Under our defined assay conditions, the non-formulated phytases reached their activity maximum at the following temperatures: A. fumigatus and A. niger phytase at 55 °C, A. terrreus CBS and A. nidulans phytase at 45 °C and consensus phytase at 65 °C. Screening of the effects of different low-Mr additives revealed that polyethylene glycols enhance the thermostability of all phytases in a molecular weight-dependent fashion. The polyols ribitol, xylitol (C5 sugars) and sorbitol (C6 sugar) also improved thermostability, whereas polyols like glycerol, erythritol and mannoheptulose containing more or less carbon atoms showed only minor effects. The stabilizing effects of PEGs and polyols were concentration-dependent. The disodium salts of malonic and succinic acid led to a concentration-dependent increase in A. fumigatus and A. nidulans phytase thermostability and to stimulation of their enzymatic activity. On the other hand, the same dicarboxylates showed no effect on the enzyme activities, but increased the thermostability of A. terreus CBS, A. niger and consensus phytase. Crosslinking of the carbohydrate chains of A. fumigatus and consensus phytase using sodium periodate and adipic acid dihydrazide resulted in the formation of oligomeric forms which may explain the observed enhancement of thermostability up to 70 °C for A. fumigatus phytase and 80 °C for consensus phytase. In summary both strategies (low-Mr additives and chemical crosslinking) added value to thermostabilize fungal phytases and their future application will have a high impact on the quality of resulting feed enzyme products.