The oligomerization of ethene over H-ZSM-5 has been used as a model reaction for studying coke formation in two different microbalance reactors, a conventional microbalance and a new oscillating TEOM 1500 microbalance reactor. Significant gradients prevailed in the catalyst bed in the basket of the conventional microbalance reactor even with this moderately rapid and exothermal reaction, making it difficult to obtain true kinetic information. In the oscillating microbalance reactor the reactants are forced to flow through the catalyst bed in the same manner as in a fixed-bed reactor and kinetic data could be obtained in the absence of transport limitations in the catalyst layer.