Abstract. Mobile devices provide a variety of ways to access information resources available on the Web and a high level of adaptability to different aspects (e.g., device capabilities, network QoS, user preferences, location, an so on) is strongly required in this scenario. In this paper we propose a technique for the efficient adaptation of Web Information Systems. The approach relies on special rules that allows us to specify, in a declarative way, how to generate, in an automatic way, the adaptation specifications that satisfy the requirements of a given context. Rules are classified in a set of clusters and a matching relation between clusters and context requirements is defined. The technique of rule evaluation and clustering guarantee that different contexts and orthogonal requirements of adaptation, possibly not fixed in advance, can be efficiently organized and taken into account in the adaptation process.