A transient hot-wire apparatus for absolute measurements of the thermal conductivity of gases has been constructed and tested with argon and methane. The apparatus operates at temperatures from about 300 to 473 K and pressures up to 60MPa. To evaluate the capability of the apparatus. measurements were per-formed (i) with argon along five isotherms at temperatures between 300 and 425 K with pressures up to 15 MPa. and (ii) with methane along seven isotherms at temperatures between 290 and 360 K with pressures up to 15 MPa. The experimental thermal conductivity data and the results of their low-density analysis are compared with results of other investigators. The total experimental uncertainty of the measurements is estimated to be +/- 0.00 1 x p in pressure and +/- 0.005 K in temperature, The relative uncertainty of the measurements at 95% level is estimated to be 1.2% for argon and 0.7% for methane. The reproducibility of the results measured by the standard deviation from the polynomial lambda(rho) relation along isotherms is 0.3% for argon and 0.2% for methane.