Delivering added value public e-services often requires cooperation between two or more government departments. In general, such cooperation is ensured by implementing interoperability between automated business processes. In spite of the existence of several works interested in public online services in general, only few of them were focused on measuring interoperability degree, whether used or required, to deliver them. In this context, the objective of this paper is to propose a new quality indicator for interoperability assessment of public integrated e-services. This indicator takes into account three main aspects: interoperation potentiality, interoperation compatibility and operational performance. Finally, to better illustrate how to use the proposed measurement method, we present a practical example of integrated public e-service. This work takes the case of a national library which proposes an online registration service.