The temp. jump relaxation method was used for kinetic measurements in the gas phase. The system N2O4 .dblarw. 2NO2 was studied. Fast temp. jumps were induced by IR absorption from a CO2 laser. Temp. jumps of <1 K led to readily detectable concn. changes which were monitored by UV absorption in a simple single shot expt. krec(Ar) is 1.05 * 1010 cm3-mol-1-s-1 at PAr = 1 bar and T = 258 K, in agreement with recent exptl. data.