A new method for measuring the rates of heterogeneous catalytic and possibly other gas-solid reactions is introduced in this paper. It is based on measuring the "flux response" of a reactor. The flux response of a continuous-flow sas-solid reactor is defined as the way the net rate at which molecules leave the reactor changes when some input, or other, variable is changed. The flux response depends on the combined effect of absorption, reaction and desorption of the gases in the reactor. Apparatus for measuring the flux response is described. The apparatus design is based on that of apparatus used in the sorption-effect method for measuring gas adsorption. Modifications to that apparatus to make it suitable for investigating the flux response of a catalytic micro-reactor are reported. Flux-response experiments have been carried out using the catalytic decomposition of methanol over platinized alumina as the demonstration reaction. The flux response measurements were complemented by mass spectrometric analysis of the reactor effluent. The experiments reported here confirm that the flux response can disclose some of the detail of the adsorption-reaction-desorption mechanism of heterogeneous catalysis. Conversions are calculated from the experimental flux responses.