The enlarging market of portable devices and wire-less networks stimulates the provisioning of mobility-enabled Internet services with differentiated levels of Quality of Service (QoS). The finally supplied QoS level can greatly differ from the negotiated one because of the very variable availability of network resources. This significantly impacts on the pricing policies and, consequently, on the definition and realization of fair accounting strategies. The paper claims the need of middleware solutions able to evolve dynamically depending on user/terminal mobility to monitor, control and register the finally supplied QoS level directly within the network localities where users/terminals move to. The paper presents the design and implementation of the Active middleware for Quality-aware Accounting of Mobile services (AQuAM) that is capable of accounting final users for the QoS level finally supplied during service provisioning, in contexts of user/terminal mobility.