The temporal evolution of the composition of the products collected from a slurry reactor during Fischer-Tropsch synthesis over a Co/Al2O3 catalyst is studied, in order to rationalize the time required for a complete renewal of the liquid phase used to initially dilute the catalyst. The transients are well described by a simple equation with no assumption on the phase repartition of the hydrocarbons in the reactor, which also provides an estimate of the time constant of the system. The mathematical model can be used to predict the time necessary to reach steady state of the composition of the reactor outlet and hence collect reliable data for product selectivity in a slurry reactor. The change in the composition at the outlet of a fixed bed reactor after step changes of a co-fed liquid olefin is also approximately analyzed: it is shown that the transient is much faster for a fixed bed reactor than for a slurry reactor. (C) 2003 Elsevier Science (USA). All rights reserved.