Nonsteady-state kinetics was used for the detn. of the reaction mechanism of a heterogeneous catalytic reaction, and the characteristic shape of the time evolution curves of the concns. at the outlet of the reactor was assocd. with rate-limiting steps. Addnl. in-situ measurements of surface intermediates were correlated with those response curves to get a phys. meaning to the detd. kinetic parameters. An exptl. setup was developed to investigate simultaneously the surface and gas-phase concns. That the knowledge of the reaction mechanism and the identification of the active sites on the catalyst are of great use for reaction engineering was exemplified with the methylation of catechol where the catalyst was selectively modified to increase the yield of 3-methylcatechol from 25 to nearly 70%. [on SciFinder (R)]