A gas-controlled heat-pipe is used at the Italian National Research Institute of metrology (INRIM) to realise the mercury (liquid + vapour) equilibrium from (450 to 700) K. The pressure in the heat pipe is controlled between (5 and 200) kPa by means of an innovative computer-driven system having a pressure sensitivity of a few parts per million. The vapour pressure of 0.9999999 pure mercury has been measured between (500 and 665) K. A standard pressure balance and a standard platinum resistance thermometer (SPRT) calibrated at the ITS-90 fixed points, were used. The experimental data obtained are fitted to a semi-empirical relationship between vapour pressure and temperature. The basic features of gas-controlled heat pipes and the metrological quality of the temperature and pressure measurements available in the laboratory, allowed the determination of the mercury vapour pressure at the level of a few parts in 10(-5). The paper reports the acquired data and the evaluated relationship, together with the uncertainty budget of the associated measurements. A complete description of the mercury heat-pipe, the gas controlled system, the measuring instruments involved and the temperature and pressure data acquisition and processing is also presented. The relationship between temperature and pressure of the mercury liquid-vapour transition has then been derived and it is here reported together with the mathematical and experimental uncertainty budgets. An accurate comparison with the existing data is also reported. (C) 2009 Elsevier Ltd. All rights reserved.