The usual explanation of how a solid catalyst promotes a gas-phase reaction is that adsorption is followed by reaction and desorption. This composite process may be followed by measuring the flow-rate changes (the flux response) caused by adsorption, reaction and desorption in a continuous-flow reactor. The mathematical modelling of the flux responses of gas-solid catalytic reactors is discussed. A numerical simulator has been developed that allows flux responses to be calculated. Examples are given and the simulator is tested against an analytical special case. The salient points of simulated flux responses are similar to those of preliminary experimental flux responses. The combination of the mathematical model with a specially designed series of experiments could allow reaction, adsorption and mass-transfer rate coefficients to be determined.