An apparatus based on the coaxial cylinder method has been used to measure the thermal conductivity of fluids in gaseous phase up to 700°C and 1000 bars. After an evaluation of the various errors and corrections, the accuracy of the measurements has been estimated better than 2.5% up to 450°C. Experimental results are given for argon up to 700°C and 1000 bars, helium up to 500°C and 1000 bars, hydrogen at 30°C as a function of the pressure and at 100 bars as a function of the temperature, nitrogen up to 500°C and 1000 bars, carbon dioxide up to 700°C and 1000 bars, methane and ethane up to 450°C and 1000 bars. For all these gases, the excess of the thermal conductivity has been represented as a function of the density and ( ∂P ∂T ) ρ These results have been compared with the existing theories of dilute and dense gases.