The residence time distribution (RTD) was obtained in a tubular reactor with mean residence times of the order of milliseconds by using an infrared laser absorption technique. The reactor was operated at room conditions with a nitrogen or helium gas flow, the tracer used was methane. The fast response of the measuring device allowed a qualitative analysis of the micromixing in the flow. An axial dispersion model was used to describe the RTD of the reactor. It was found that the values for the dispersion coefficient were strongly influenced by the radial velocity profile in the reactor.