A flow calorimeter with an adjustable throttle has been used to measure the isothermal Joule-Tnomson coefficient of water vapour at temperatures from T = 313.00 K to 413.19 K. At each temperature measurements made over a range of pressures were extrapolated to zero pressure to obtain the quantity phi (o), = (B - TdB/dT), where B is the second virial coefficient. At temperatures above 330 K the measurements are in good agreement with the work of Collins and Keyes (1937), and if their value at T = 312.09 K is corrected by 3.3 per cent, the amount by which their measurement of the heat capacity of steam at this temperature is in error, the agreement is within the combined experimental error over the whole temperature range. Values of the zero pressure isothermal Joule-Thomson coefficient phi (o) derived from the measurements are compared with an equation for B(T) proposed by Gallagher which is consistent with the 1984 NBS/NRC steam tables, an equation for B(T) proposed in 1988 by Hill and MacMillan, and with values of B(T) derived from the 1995 IAPWS equation of state for steam of Pruss and Wagner. The new measurements are found to be in agreement with the equation of Hill and MacMillan to within the experimental error. (C) 2000 Academic Press.