The heat conductivity of steam has been measured by Timroth and Vargaftig (1940) from approximately 100 to 550 C and to pressures of 250 atm, using the hot-wire type of conductivity cell, wherein the wire of pure platinum serves as thermal emitter, and its electrical-resistance change provides the temperature indication. The new measurements in the present paper were obtained using a concentric cylinder type of conductivity cell in which the heater and thermometric parts were isolated from contact with the material whose conductivity was being measured. Vargaftig had published (1937) measurements of the heat conductivity of nitrogen in substantially the same cell used for steam. The new measurements on nitrogen are in tolerable agreement with the Vargaftig measurements, but in the case of steam the new measurements are considerably lower in magnitude. A comparison of the two sets of steam data indicates some peculiarities of temperature trend in the Timroth and Vargaftig data which find no correspondence in the data of any other substance, and also differs from the trend relative to the new data. A formulation of the steam and nitrogen data is given in terms of pressure and temperature as independent variables.