Almost without exception, heterogeneously catalysed gas-solid reactions conform to the A definition of complex reactions in that they proceed through a series of parallel and sequential elementary reactions. The catalysts themselves are usually multicomponent and often contain a high surface area metal which would be pyrophoric if exposed to air, while the reacting gaseous medium usually contains two or more components. The situation in which the surface atomic composition, total surface area and metal surface area are measured in one or, sometimes, two pieces of equipment while the reaction rate measurements are carried out in another, with the adsorption and desorption kinetics being measured in yet another piece of equipment, is clearly less than ideal. Using this approach it is extremely difficult to relate the results of one set of experiments to another and virtually impossible to extrapolate these measurements to the conditions obtained during steady state reaction. In situ measurement is obviously the ideal approach. This paper describes methods for the in situ determination of: (i) total surface area; (ii) metal surface area; (iii) adsorption kinetics and sticking probabilities; (iv) desorption kinetics; (v) surface reaction kinetics; (vi) surface adsorbate and atomic compositions, obtaining under reaction conditions; (vii) the structure of the adsorbates; (viii) steady state rates. The techniques involved include isothermal gas adsorption chromatography and temperature programmed methods (e.g. temperature programmed desorption, reaction, reduction, oxidation, surface reaction, x-ray photoelectron spectroscopy and infrared spectroscopy).