This paper addresses the problem of formalizing service level specifications (SLSs) as a first step to simplify and automate the configuration and management of multiservice IP networks. A formal representation of SLSs will allow their automatic validation and processing, fostering the dynamic negotiation of SLSs and the interoperability among service management entities. In this way, taking advantage of XML extensibility and portability, a Schema is presented describing XML SLSs sections and their contents. In addition, an XML validator tool was built to check if SLSs are correctly specified. An XML SLS for an IP telephony service is used to exemplify this proposal expressiveness.