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Grid computing systems need to achieve high levels of Quality of Service (QoS) necessary for enterprise applications in science and industry. A Service Level Agreement (SLA) specification provides a formal method for describing QoS requirements. WS-Agreement is a language and protocol designed for advertising the capabilities of providers and creating agreements based on initial offers, and for monitoring agreement compliance at runtime . The definition of the protocol is very general and allows for the negotiation of QoS in any Web service enabled distributed system. However, the WS-Agreement specification does not contemplate the possibility of changing an agreement at runtime. Therefore, the challenge is to provide extensions of WS-Agreement and its semantics in order to make agreements robust and more long-lived to individual term violations. This paper will show extensions of the WS-Agreement specification to support the dynamic nature of SLAs by allowing the possibility of SLA renegotiation at run time.