A basic requirement for the production of large power plant components from ferritic/martensitic 9-12% Cr steels is good weldability. Weldments in these steels are often reported as the weak spots. In this work the weldability of a creep resistant 9% Cr steel is discussed. Different methods are utilized to characterize the microstructural evolution during different welding cycles and the following post weld heat treatment, as well as the resulting mechanical properties. Heat affected zone (HAZ) simulation using a thermo-mechanical testing device GLEEBLE 1500 is performed to study the microstructural evolution and changes in the mechanical properties in the different parts of the HAZ. Specimens exposed to peak temperatures higher than 1150°C showed a minimum of impact toughness after post weld heat treatment (PWHT). In situ X-ray diffraction experiments with synchrotron radiation are used to observe phase transformations during heating to elevated temperatures, where delta ferrite formation was observed at temperatures higher than 1250°C.