The integration of the widely spread 3rd Generation (3G) Universal Mobile Telecommunication System (UMTS) with the Wireless Local Area Networks (WLANs) has recently attracted considerable attention. However, addressing seamless and transparent handoff during mobility between these networks is still the biggest challenge to overcome. This paper evaluates the components of the handoff delay for the 3GPP UMTS/WLAN interworking architecture. We analyze the handoff delay components due to security procedures, Dynamic Host Configuration Protocol (DHCP), Mobile IP, and the one-way or data transfer delay. This study utilizes a simulation model comprised of network of queues to model the integrated networks at the packet level taking into account both signaling traffic and background loads. Simulation results show that the handoff delay component due to security procedures is the dominant relative to other components under the considered network parameters. In addition, results also reveal that this former component specifically, and the total handoff delay in general, are very sensitive to background load in the network.