The validation of a product interface is often a critical issue in the design process. Virtual reality and mixed reality (MR) are able to enhance the interactive simulation of the product human–machine interface (HMI), as these technologies allow engineers to directly involve end users in the usability assessment. This paper describes a MR environment specifically addressed to the usability evaluation of a product interface, which allows the simulation of the HMI behaviour using the same models and the same software employed by engineers during the design phase. Our approach is based on the run-time connection between the visualisation software and the simulators used for product design and analysis. In particular, we use Matlab/Simulink to model and simulate the product behaviour, and Virtools to create the interactive MR environment in which the end user can test the product. Thanks to this architecture, any modification done on the behaviour models is immediately testable in MR.