[en] A new apparatus for measuring both thermal conductivity and thermal diffusivity of fluids at high temperatures is described. The technique employed is that of the transient hot wire. Measurements are made with a 12.7 μm diameter platinum wire at times of up to 1 second. The data acquisition system is controlled by a microcomputer and includes several programmable digital voltmeters. The hot wire and a shorter compensating hot wire are arranged in different arms of a Wheatstone bridge. The cell containing the core of the apparatus is designed to accommodate pressures from near zero to 70 MPa and temperatures from 0 to 500⁰C. For thermal conductivity, the precision of the new system is expected to be around 0.3% and the accuracy 1.0%. For thermal diffusivity the accuracy is estimated to be around 5%. From the two variables measured, one can obtain values of the specific heat, C/sub p/, of the fluid, provided that the density is either measured, or available through an equation of state