This thesis deals with the temperature stability of Xanthan Gum in different solutions. Different samples, provided by a manufacturer, are compared with another product, provided by a different manufacturer. For this comparison an intensive literature and laboratory study was conducted to evaluate the behaviour of the biopolymer Xanthan Gum at elevated temperatures. A high pressure/high temperature rheometer was used to characterise temperature stability of different Xanthan Gum samples. In addition, a new testing procedure was developed to evaluate the viscosity of the fluids at wellbore conditions with a special emphasize on the repeated temperature loading due to circulation of the mud (CTLT- Cyclic Temperature Load Test). Furthermore the impact of elevated temperatures on the viscosity is measured for a standard Glycol- Potassium Carbonate mud and for a Bentonite mud. The results and their impact on some currently used hydraulics calculations are qualitatively evaluated. The tests showed that different Xanthan Gum samples show quite different low shear rate viscosities and differing temperature stability in Standard Tap Water and Seawater. In the Glycol- Potassium Carbonate mud, however, properties of the tested Xanthan Gum samples do not have a strong influence on temperature stability and cyclic temperature loading. On pressure drop within pipes, the temperature dependence of viscosity has a significant influence.