One of the typical features of distributed systems is the heterogeneity of its components (e.g. geographical spreading and different platform architectures), leading to interoperability issues. Many of these are handled by generic middleware-based solutions.This paper presents an analytic model of the impact of using such middleware on the overall system performance. Specifically, a Layered Queueing Network is described that models a client/server system using CORBA as a middleware system offering location transparency. The response times estimated from the model are compared to the measured response times for a growing number of clients, in order to assess the accuracy of the model and the values of the parameters in the model. This model can then be used for designing a distributed application, before the entire system is installed or even fully implemented.