A variation of total pressure may be used to check for external mass transfer limitation in heterogeneous catalytic experiments. This is particularly interesting for microchannel reactor studies where a variation of flow rate is not suitable. The pressure variation approach is demonstrated for the steam reforming of methanol in a microstructured reactor. The result is verified by another method, viz. the variation of the carrier gas. ne two methods are in good agreement. Other implications of a pressure variation in microchannels are discussed.