In this paper, the authors present an implementation of a plug-n-play Web sensor based on a Web service approach, utilizing the extensible markup language, which is an open-source software, to exchange information and adopting a low-cost hardware. After an introduction to the Web service technology, the problem of the dynamic services and one possible solution will be presented. The proposed solution allows us to have, at all times, an overview of the state of connected sensors and of the available services. Thus, when a new sensor is inserted in the network, its services are automatically "published" without reconfiguring the net; this is important with sensor networks, which are frequently upgraded. The sensor is presented in the framework of a hierarchical distributed system for metrological monitoring applications. Particular attention is devoted to synchronization problems among all composing devices of the proposed distributed system, taking into account the characteristics of main synchronization standards. Measurement results supplied by the sensors are made available as Web services so that all users can build up their own applications. To show some preliminary results of the proposed distributed monitoring network, the monitoring of rms values in three-phase power networks was analyzed. A proper synchronization protocol was adopted in order to synchronize the different hardware sections of a sensor to perform a particular measurement.