Sizewell D is Nuclear Electric＇s latest power station, and the Pressurised Water Reactor (PWR) design on which it is based represents a ＇first＇ for the UK. One of the integral components of the plant is the heating, ventilation and air-conditioning (HVAC) system, which performs a contamination control and gaseous waste management function for the site. During the commissioning of Sizewell B Power Station the extract systems of the HVAC plant underwent a procedure known as ＇fingerprinting＇. This entailed the characterisation of the facilities provided to test the filtration plant during its lifetime. The assessment of their adequacy was then used to identify necessary modifications and/or to propose the manner in which future in situ performance testing would be carried out. The paper outlines the basic principles and procedure that was used to ＇fingerprint＇ test systems during the commissioning of Sizewell B. A specific example is presented to demonstrate the process.