We present a technique for the canonicalization of MPEG-21 REL licenses based on the semantics of their elements. Although the representation format used in these licenses is XML, our canonicalization is not based on the canonical XML specification because it is concerned only with the syntactic level of XML, and we want to define a canonicalization that produces the same result for all licenses that are semantically equivalent. In this paper we list the main rules that we have defined for this semantic canonicalization focused on the specific case of MPEG-21 REL licenses. Finally, we discuss a possible generalization that could be used for specifying semantic canonicalization rules in any other XML application. However, it must be realized that defining a single and universal semantic XML canonicalization is a virtually impossible task, because in general it will be infeasible to consider every possible semantics.