In this paper, we present experimental results of the deployment of a wireless sensor network for inventory and localization of mobile assets in an indoor office building. The system is based on the IEEE 802.15.4 standard at 2.4 GHz and the localization scheme is based on RSSI measurements between mobile tags and an infrastructure of reference nodes located in the ceiling of the building. We present an overview of the system, the deployment scenario in the building and we evaluate the localization performances in terms of the error distance vector mean and standard deviation. We study the localization performance as a function of the reference node density in the building. We show that the system achieves at best a positioning accuracy below 2.5m at 95% probability.