The break-in behaviors of two similar heterogeneously catalyzed systems were studied during propylene disproportionation. A cobalt oxide-molybdena-alumina (CoO-MoO₃—Al₂O₃) catalyst was found to undergo a rapid increase in the catalytic activity during the first 100 seconds of contacting with the reactants and this was the order of magnitude of the mass transport delays. A slower first order process was observed with a break-in period of about 30 minutes, and this process is reversed by inert purge. The effects of the products of propylene disproportionation on the activity of a tungsten oxide on silica (WO₃-SiO₂) catalyst were studied by using a split-bed reactor and using mixtures containing low concentrations of products in propylene as feed. In both experiments the products were found to have some suppressing effects on the activity of this catalyst. The activity of the WO₃-SiO₂ catalyst was reduced to one tenth of the original by adding 0.02N NaOH aqueous solution to the catalyst and drying at 120°C for twelve hours. Dosing hydrogen (a reductant) and oxygen (an oxidant) to the fully activated WO₃-SiO₂ catalyst individually or alternatively reduced the activity of this catalyst to a point where another break-in period was needed to restore full activity. The break-in period of cooling and reheating the steady-state WO₃-SiO₂ catalyst in stagnant propylene was much shorter than that observed in propylene flow. / Master of Science