This thesis present a highly sensitive silicon microreactor and examples of its use in studying catalysis. The experimental setup built for gas handling and temperature control for the microreactor is described. The implementation of LabVIEW interfacing for all the experimental parts makes automated experiments and data collection possible. An argon ush at the O-rings (used to interface the silicon microreactor with the gas system), which was developed, is presented. It enables experiments with temperatures up to 400., and up to 500. for short periods of time. The CO oxidation reaction on platinum thin _lms is used to test the sensitivity of the microreactor. Activity is shown to be measurable for as little as a 15 _m2 platinum, with an activation energy of _1 eV. A study of the light o_ phenomenon on platinum, showing light o_ at room temperature in gas mixtures of CO and O2 with a large oxygen surplus, is presented. The e_ect of pretreating the catalyst, CuZnO, in a mixture of H2 and CO before methanol synthesis, is presented. Transient increased methanol production is seen after pretreatment, with a maximum in the transient for a pretreatment with a one to one CO to H2 ratio. The highly active state of the catalyst after pretreatment in a CO and H2 mixture is shown to have transient methanol synthesis capabilities at 60.. Estimates of the area of the catalytic surface, is obtained using formate temperature programmed desorption measurements. From these, the possibility of adsorbates readily converted to methanol as the source of the transient increase in methanol production, is eliminated. A study of mass selected ruthenium nanoparticles from a magnetron-sputter gas-aggregation source, deposited in microreactors, is presented. It is, shown that CO methanation can be measured on the mass selected nanoparticles in the microreactor. A parameter study shows negative reaction order in the CO concentration and apparent acivation energies between 0.8 and 1.2 eV depending on reaction conditions. Temperature programmed reaction studies in H2m shows di_erent forms of carbon growth on ruthenium nanoparticles subjected to methane and CO at 250.. The use of the microreactor for photocatalysis and the development of a new two phase microreactor intended for photoelectrocatalysis is described. A single experiment of water electrolysis, with simultaneous measurement of H2, O2, and the cell current, is presented.