Reports the results of measurement of ventilation efficiency in a full scale model of a small office with a diffuse air supply, which can be used to improve ventilation efficiency and therefore reduce air supply rates. Steady state and transient definitions of ventilation efficiency are given. Finds that ventilation efficiency depends on air flow rate and heat gain in the office. Higher air flow rates give lower pollutant concentration in the ocupied zone but also more mixing between the occupied and under-ceiling zones. Increasing heat gains give higher pollutant concentrations in the occupied zone and clearer demarcation between cool and warm zones (clean and polluted zones). A comparison is also made between diffuse air supply and conventional diffusers.