We present a comprehensive formal semantics for a UML state machine kernel which also considers the use and manipulation of complex structured data. We refer to the UML standard Version 2.1.1 which was published in year 2007. There has been no work that completely integrates complex structured data into a UML state machine semantics. We follow a ”semantics-first” approach (in opposite to a ”complete-notation-first” approach) in which we consider a sound basic kernel of the UML state machine notation, and extend this kernel only ater a thorough investigation of the impacts. We define an operational semantics which is intended to be implemented in a standard programming language. Currently we use such an implementation to automatically generate test cases out of a state machine specification. This document is intended to be adapted if necessary. We will indicate that by the version number given above, whereat the major version number indicates changes of the considered subset and the minor version number indicates adoptions and corrections.