The thermal conductivity of the dissociating system N₂O₄ [A: r harp over l] 2NO₂ has been measured at 35 °c and 50 °c over a range of pressures by a precision hot-wire method. The present theory of relaxation effects in the heat conductivity of a chemically reacting gas mixture has been critically tested. It has been found, on varying the gas pressure, that the theoretical estimate of this effect holds only up to a certain upper limit of pressure, beyond which the observed effect becomes insignificant compared with the theoretical predictions. Moreover, the authors' recent suggestion of a probable effect of chemical reaction upon diffusion between the various chemical species has been well confirmed by the experiments.