Service-orientation enables cooperation between multiple organizations and has become a solution of choice to tackle the complexity of ubiquitous computing. The very nature of ubiquitous applications implies a need for dynamic solutions. Service oriented computing provides support for these dynamic applications. However, current solutions overlook important aspects, particularly when dynamically substituting services. Some applications may be mission-critical and therefore the disruption of a particular service could be harmful. Otherapplications may tolerate the disappearance of a service if the service returns within a predefined amount of time. Hence, guarantees regarding availability of services that compose an application are required. In this paper we propose to take into account service disruptions through service level agreements for dynamic service-oriented applications.